Polyacrylic acid (salt)-type water absorbent resin and method for producing of same

ABSTRACT

A process for producing a water absorbent resin is provided with which it is possible to efficiently and inexpensively obtain a surface-crosslinked water-absorbing resin having excellent material properties, while ensuring high productivity. In continuous production scaled up to a large scale (in particular, 1 t/hr or more), the physical properties are improved and stabilized (for example, a reduction in standard deviation of the physical properties) by a surface crosslinking treatment to further improve absorption against pressure (AAP) and saline flow conductivity (SFC). The process for producing a polyacrylic acid (salt)-type water absorbent resin includes a surface treatment step in which after addition of a surface-crosslinking agent, a crosslinking reaction is conducted in a transverse type continuous stirring apparatus having stirring means including a feeding inlet and a discharging outlet of a water absorbent resin and one or more rotary shafts having with a plurality of stirring discs, and heating means, at a stirring-power index of the device of 3-15 W·hr/kg.
 
(Stirring-power index)=((power consumption of apparatus at the time surface treatment)−(power consumption at the time of idling))×average retention time)/(treatment amount per unit time×average retention time).

TECHNICAL FIELD

The present invention relates to a polyacrylic acid (salt)-type waterabsorbent resin and a method for producing the same. More particularly,the present invention relates to a polyacrylic acid (salt)-type waterabsorbent resin having high water absorption rate (CRC), high waterabsorption against pressure (AAP), and high liquid permeability (SFC)and containing little water Extractables and a method for producing thesame.

BACKGROUND ART

A water absorbent resin (Super Absorbent Polymer; abbreviated as SAP)has been used in a wide range of uses for sanitary materials such aspaper diapers, sanitary napkins, incontinence products for adults, andthe like, and uses for water retention agent for soil, owing toproperties that the resin can absorb a large quantity of a water-basedliquid several times to several hundred times as much as the mass ofitself and has been manufactured and consumed in large quantities.

In general, a water absorbent resin is produced by polymerizing anaqueous solution containing a hydrophilic monomer and a crosslinkingagent to obtain a hydrous gel-like polymer, drying the gel polymer, andsurface-crosslinking the dried product. The physical properties such aswater absorption against pressure (AAP) and liquid permeability (GBP,SFC) of the above-mentioned water absorbent resin are improved bysurface-crosslinking step. The surface-crosslinking step is commonly astep of providing a highly crosslinked layer in the vicinity of thewater absorbent resin surface by causing reaction of the water absorbentresin with a surface-crosslinking agent or a polymerizable monomer.

Various kinds of surface-crosslinking agents reactive on a functionalgroup of a water absorbent resin (particularly, carboxyl group) areproposed as a surface-reforming method of such a water absorbent resinand examples known as the surface-crosslinking agents are oxazolinecompounds (Patent Document 1), vinyl ether compounds (Patent Document2), epoxy compounds (Patent Document 3), oxetane compounds (PatentDocument 4), polyhydric alcohol compounds (Patent Document 5), polyamidepolyamine-epihalo adducts (Patent Documents 6, 7), hydroxyacrylamidecompounds (Patent Document 8), oxazolidinone compounds (Patent Documents9, 10), bis- or poly-oxazoline compounds (Patent Document 11),2-oxotetrahydro-1,3-oxazolidine compounds (Patent Document 12), alkylenecarbonate compounds (Patent Document 13), and the like. A techniqueusing a specified surface-crosslinking agent (Patent Document 14) isalso known.

Techniques also known as the surface-reforming method other than themethod carried out by a surface-crosslinking agent are a technique ofsurface-crosslinking by polymerizing a monomer in the vicinity of thewater absorbent resin surface (Patent Document 15) and techniques ofradical crosslinking with persulfuric acid salts etc. (Patent Documents16, 17). Techniques of reforming water absorbent resins by heatingwithout using a surface-crosslinking agent (Patent Documents 18, 19),which is different from common surface-crosslinking treatment, are alsoknown.

A technique of using an additive in combination for mixing asurface-crosslinking agent is also proposed and examples known as theadditive are water-soluble cations such as aluminum salts and the like(Patent Documents 20, 21), alkali (Patent Document 22), organic acidsand inorganic acids (Patent Document 23), peroxides (Patent Document24), and surfactants (Patent Document 25).

Not only the chemical methods but also many surface treatment methodsusing apparatuses and reaction conditions have been proposed. Examplesknown as a method using an apparatus are techniques using a specifiedmixing apparatus as a mixing apparatus for a surface-crosslinking agent(Patent Documents 26 to 29) and techniques using a heating apparatus forcausing reaction of a water absorbent resin and a surface-crosslinkingagent (Patent Documents 30, 31) and the like.

There is also a technique for controlling an increase in heatingtemperature for causing reaction of a water absorbent resin and asurface-crosslinking agent (Patent Document 32) in improvement of thereaction condition aspect. In a heating step, techniques known are atechnique of carrying out surface-crosslinking twice (Patent Document33), a technique of controlling particle size by drying a waterabsorbent resin, thereafter carrying out a second heat drying step, andfurther carrying out surface-crosslinking (Patent Document 34), atechnique of defining oxygen partial pressure (Patent Document 35),techniques of defining the spraying conditions and dew points (PatentDocuments 37, 38), techniques of defining the mixing conditions oftreatment liquids (Patent Documents 39, 40), and a technique payingattention to a cooling step (Patent Document 41).

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: U.S. Pat. No. 6,297,319-   Patent Document 2: U.S. Pat. No. 6,372,852-   Patent Document 3: U.S. Pat. No. 6,265,488-   Patent Document 4: U.S. Pat. No. 6,809,158-   Patent Document 5: U.S. Pat. No. 4,734,478-   Patent Document 6: U.S. Pat. No. 4,755,562-   Patent Document 7: U.S. Pat. No. 4,824,901-   Patent Document 8: U.S. Pat. No. 6,239,230-   Patent Document 9: U.S. Pat. No. 6,559,239-   Patent Document 10: U.S. Pat. No. 6,503,979-   Patent Document 11: U.S. Pat. No. 6,472,478-   Patent Document 12: U.S. Pat. No. 6,657,015-   Patent Document 13: U.S. Pat. No. 5,409,771-   Patent Document 14: U.S. Pat. No. 5,422,405-   Patent Document 15: US Patent Application Publication No.    2005/048221-   Patent Document 16: U.S. Pat. No. 4,783,510-   Patent Document 17: EP Patent No. 1824910-   Patent Document 18: U.S. Pat. No. 5,206,205-   Patent Document 19: EP Patent No. 0603292-   Patent Document 20: U.S. Pat. No. 6,605,673-   Patent Document 21: U.S. Pat. No. 6,620,899-   Patent Document 22: U.S. Pat. No. 7,312,278-   Patent Document 23: U.S. Pat. No. 5,610,208-   Patent Document 24: US Patent Application Publication No.    2007/078231-   Patent Document 25: US Patent Application Publication No.    2005/029352-   Patent Document 26: U.S. Pat. No. 5,140,076-   Patent Document 27: U.S. Pat. No. 6,071,976-   Patent Document 28: US Patent Application Publication No.    2004/240316-   Patent Document 29: WO No. 2007/065840 pamphlet-   Patent Document 30: US Patent Application Publication No.    2007/149760-   Patent Document 31: Japan Patent Application Publication No.    2004-352941-   Patent Document 32: U.S. Pat. No. 6,514,615-   Patent Document 33: U.S. Pat. No. 5,672,633-   Patent Document 34: WO No. 2009/028568 pamphlet-   Patent Document 35: US Patent Application Publication No.    2007/0293632-   Patent Document 36: U.S. Pat. No. 6,720,389-   Patent Document 37: U.S. Pat. No. 7,183,456-   Patent Document 38: US Patent Application Publication No.    2007/161759-   Patent Document 39: US Patent Application Publication No.    2006/057389-   Patent Document 40: EP Patent No. 0534228-   Patent Document 41: U.S. Pat. No. 7,378,453

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

It is difficult to satisfy demands from users for physical propertiessuch as water absorption against pressure and liquid permeability of awater absorbent resin only by a surface-crosslinking technique, althoughthere have been proposed many of surface-crosslinking agents (see PatentDocuments 1 to 13) and their combination use (see Patent Document 14),auxiliary agents for surface-crosslinking (see Patent Documents 20 to25), their mixing apparatuses (see Patent Documents 26 to 29) andheating apparatuses (Patent Documents 30, 31), and also various kinds ofconditions (see Patent Documents 32 to 41). Along with change of asurface-crosslinking agent and use of a new auxiliary agent, it may besometimes accompanied with an increase in cost, a decrease in safety,deterioration of other physical properties (e.g., coloration), and thelike in some cases. Although causing an effect to a certain extent in asmall scale or batch type production in an experimental laboratorylevel, the above-mentioned means may not sometimes show so mucheffective in an industrial scale (e.g., 1 t or more per unit hour) suchas large scale continuous production as compared with that in a smallscale.

The present invention has been completed in terms of the problems, andan object of the present invention is to provide a method for producinga water absorbent resin which is excellent in physical properties andsurface-crosslinked efficiently at a low cost while assuring highproductivity.

Solutions to the Problems

The inventors of the present invention have made investigations on asurface-crosslinking step for solving the above-mentioned problems andconsequently have solved the problems by carrying out heating treatmentwith a specified apparatus and with also a specified stirring powerindex in the surface treatment step after addition of asurface-crosslinking agent.

That is, the present invention provide a method for producing apolyacrylic acid (salt)-type water absorbent resin, comprising a step ofpreparing an aqueous monomer solution of an acrylic acid (salt), a stepof continuously polymerizing the aqueous monomer solution, a step offinely shredding a hydrous gel-like crosslinked polymer during or afterpolymerization, a step of drying the obtained particulate hydrousgel-like crosslinked polymer, and a surface treatment step of adding andreacting a surface treatment agent to and with the dried water absorbentresin powder, wherein crosslinking reaction is carried out at a stirringpower index of 3 to 15 W·hr/kg in a transverse type continuous stirringapparatus having stirring means including a feeding inlet and adischarging outlet of a water absorbent resin and one or more rotaryshafts having a plurality of stirring discs, and heating means after theaddition of the surface-crosslinking agent in the surface treatmentstep.

Herein, the stirring power index is defined as (stirring powerindex)=((power consumption of apparatus at the time surfacetreatment)−(power consumption at the time of idling))×average retentiontime/(treatment amount per unit time×average retention time).

Effects of the Invention

According to the present invention, in continuous production in a largeindustrial scale (particularly, treatment amount of 1 t/hr or more), thephysical properties (e.g., water absorption against pressure and liquidpermeability) can be improved after surface-crosslinking and thefluctuation of physical property (standard deviation) can be narrowed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing one example of theconfiguration of a heating apparatus or a cooling apparatus having abiaxial transverse type continuous stirring apparatus and being employedin the embodiment of the present invention.

FIG. 2( a) and FIG. 2( b) are a cross-sectional view showing one exampleof a stifling disk (equipped with scraping blades) of a biaxial heatingapparatus or cooling apparatus.

FIG. 3 is a schematic view of the vertical cross section of a heatingapparatus and a cooling apparatus having a connected biaxial transversetype continuous stirring apparatus. Herein, the heating apparatus or thecooling apparatus is an apparatus having a similar shape with the sameinner area (inner volume) and can correspond to conventional techniques.

FIG. 4 is a cross-sectional view showing one example of a high speedrotation stirring type mixing apparatus usable for mixing asurface-crosslinking agent. Reference numeral 6 represents a stirringshaft, and 7 (7 a) represents stirring blades.

FIG. 5 is a cross-sectional view showing one example of a transversetype high speed rotation stirring type mixing apparatus usable formixing a surface-crosslinking agent. Reference numeral 2 represents aninner wall, 6 represents a stirring shaft, and 7 (7 a, 7 b) representsstirring blades.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the polyacrylic acid (salt)-type water absorbent resin ofthe present invention and a method for producing the same will bedescribed in detail; however, the scope of the present invention is notrestricted to the following description, and those other than thefollowing examples can be properly modified and carried out in a rangewhere the gist of the present invention is not impaired. Specifically,the present invention is not limited to each of the followingembodiments, and various modifications can be made within a range shownby the claims and embodiments carried out by properly combining eachtechnical means disclosed with different embodiments are also includedwithin the technical scope of the present invention.

[1] DEFINITION OF TERMS (a) “Water Absorbent Resin”

The “water absorbent resin” means water-swelling and water-insoluble“polymer gelling agent” and includes those having the following physicalproperties. That is, those having, as the water-swelling property, awater absorption rate under no pressure (CRC) of 5 g/g or more. CRC ispreferably 10 to 100 g/g and more preferably 20 to 80 g/g. Because ofwater-insolubility, it is required that water Extractables are in amountof 0 to 50 mass %. The water Extractables are preferably in amount of 0to 30 mass %, more preferably in amount of 0 to 20 mass %, and evenpreferably in amount of 0 to 10 mass %.

The water absorbent resin is not limited to be embodiments of 100 mass %of a polymer and may contain other additives (described below) to theextent of retaining the above-mentioned characteristics. That is, even awater absorbent resin composition having the water absorbent resin andadditives is generally named as a water absorbent resin in the presentinvention. The content of the polyacrylic acid (salt)-type waterabsorbent resin is preferably 70 to 99.9 mass % relative to the entirewater absorbent resin, more preferably 80 to 99.7 mass %, and still morepreferably 90 to 99.5 mass %. The components other than the waterabsorbent resin are preferably water in terms of the water absorptionspeed and impact resistance of powder (particles) and may include, ifnecessary, additives described below.

(b) “Polyacrylic Acid (Salt)”

The “polyacrylic acid (salt)” means a polymer containing arbitrarily agraft component and, as a repeating unit, an acrylic acid (salt) as amain component. The acrylic acid (salt) as a monomer excluding acrosslinking agent is in an amount of essentially 50 to 100% by mole,preferably 70 to 100% by mole, more preferably 90 to 100% by mole, andstill more preferably substantially 100% by mole. The acrylic acid saltas the polymer essentially contains a polyacrylic acid salt andpreferably contains a monovalent salt, more preferably an alkali metalsalt or ammonium salt, still more preferably an alkali metal salt, andparticularly preferably sodium salt. The shape is not particularlylimited; however, the polyacrylic acid (salt) is preferably particles ora powder.

(c) “EDANA” and “ERT”

“EDANA” is an abbreviation of European Disposables and NonwovensAssociations. “ERT” is an abbreviation of measurement method (ERT/EDANARecommended Test Method) of a water absorbent resin on the basis ofEuropean Standards (almost Global Standards) as defined below. In thisspecification, unless otherwise specified, the physical properties of awater absorbent resin are measured based on ERT original text (KnownDocument: revised in 2002).

(c-1) CRC (ERT441.2-02)

The “CRC” is an abbreviation for Centrifuge Retention Capacity and meanswater absorption rate under no pressure (simply sometimes referred to as“water absorption rate”). Specifically, the CRC is the water absorptionrate (unit; g/g) after 0.200 g of a water absorbent resin in a nonwovenfabric bag is freely swollen in 0.9 mass % saline solution for 30minutes and dewatered by a centrifuge at 250 G.

(c-2) AAP (ERT442.2-02)

The “AAP” is an abbreviation for Absorption Against Pressure and meanswater absorption against pressure. Specifically, the APP is the waterabsorption rate (unit; g/g) after 0.900 g of a water absorbent resin isswollen in 0.9 mass % saline solution for 1 hour under 1.9 kPa load. Inthe present invention and examples, the measurement is carried out at4.8 kPa.

(c-3) “Extractables” (ERT 470.2-02)

“Extractables” means the amount of water soluble components (dissolveamount). Specifically, measurement is carried out by adding 1.000 g ofthe water absorbent resin to 200 ml of an 0.9 mass % aqueous salinesolution, stirring the solution for 16 hours, and measuring the amountof a dissolved polymer by pH titration (unit: mass %).

(c-4) Residual monomers (ERT410.2-02)

The “residual monomers” means the amount of monomers remaining in awater absorbent resin. Specifically, the amount of monomers is a value(unit; ppm by mass) obtained by measuring, after 1.000 g of a waterabsorbent resin is charged to 200 cm³ of 0.9 mass % saline solution andthe resultant is stirred for 2 hours, the amount of monomers eluted inthe aqueous solution by using high-pressure liquid chromatography.

(c-5) PSD (ERT420.2-02)

The “PSD” is an abbreviation for Particle Size Distribution and meansthe particle size distribution measured by sieving classification. Themass average particle diameter and the particle diameter distributionwidth can be measured by the same method as in “(1) Average ParticleDiameter and Distribution of Particle Diameter” described in EuropeanPatent No. 0349240, p. 7, lines 25-43 and WO 2004/069915.

(c-6) Others

“pH” (ERT400.2-02): The “pH” means pH of a water absorbent resin.

“Moisture Content” (ERT 430.2-02) The moisture content means the watercontent of a water absorbent resin.

“Flow Rate” (ERT 450.2-02) The flow rate means the flow down speed of awater absorbent resin powder.

“Density” (ERT 460.2-02): The density means the bulk specific density ofa water absorbent resin.

(d) “Liquid Permeability”

The “liquid permeability” means the flow of a liquid flowing amongparticles of swollen gel under a load or no load. The “liquidpermeability” can be measured by SFC (Saline Flow Conductivity) or GBP(Gel Bed Permeability) as a representative measurement method.

The “SFC” is liquid permeability of 0.69 mass % physiological salinesolution in a water absorbent resin at a load of 0.3 psi. It is measuredaccording to an SFC testing method described in U.S. Pat. No. 5,669,894.

The “GBP” is liquid permeability of 0.69 mass % physiological salinesolution in a water absorbent resin under a load or free expansion. Itis measured according to a GBP testing method described in WO2005/016393 pamphlet.

(e) “Standard Deviation”

The “standard deviation” is a numeral value showing the degree ofdispersion of data and means a positive square root of the valuecalculated by totalizing the square value of the difference of the valueof data of n samples and their arithmetic average, that is, thedeviation, and dividing the total by n−1. It is used for understandingthe degree of fluctuation for the phenomenon with considerablefluctuation. In this specification, the standard deviation is employedfor digitalization of the fluctuation (deflection) for a desiredphysical value of interest.

$\begin{matrix}{\mspace{20mu}{{{{{{Data}\mspace{14mu}{of}\mspace{14mu} n\mspace{14mu}{samples}\mspace{14mu} x_{1}},x_{2},\ldots\mspace{14mu},x_{n}}\mspace{20mu}{{Arithmetic}\mspace{14mu}{average}\mspace{14mu} X}} = {\frac{1}{n}{\sum\limits_{i = 1}^{n}{Xi}}}}\mspace{20mu}{{{Standard}\mspace{14mu}{deviation}} = \sqrt{\frac{1}{n - 1}{\sum\limits_{i = 1}^{n}\left( {{Xi} - X} \right)^{2}}}}}} & \left\lbrack {{Numeral}\mspace{14mu} 1} \right\rbrack\end{matrix}$

(f) Others

In this specification, “X to Y” showing a range means “X or more and Yor lower”. Additionally, the unit of mass “t (ton)” means “Metric ton”.Further, the measurement of physical properties of a water absorbentresin is carried out under the conditions of a temperature of 20 to 25°C. (sometimes simply referred to as “room temperature” or “normaltemperature”) and a relative humidity of 40 to 50%, unless otherwisestated.

[2] A METHOD FOR PRODUCING POLYACRYLIC ACID (SALT)-TYPE WATER AbsorbentResin (1) Polymerization Step (a) Monomer (Excluding a CrosslinkingAgent)

A monomer of the present invention contains the above-mentioned acrylicacid or its salt as a main component and in terms of water absorptioncharacteristics and decrease of the residual monomers, the acid groupsof a polymer are preferable to be neutralized and the neutralizationratio is 10 to 100% by mole preferable, more preferably 30 to 95% bymole, still more preferably 50 to 90% by mole, and particularlypreferably 60 to 80% by mole, The neutralization may be carried out forthe polymer (hydrogel) after polymerization or for the monomer; however,in terms of productivity and improvement of AAP, neutralization of themonomer is preferable. Consequently, the monomer in the presentinvention includes a partially neutralized salt of the acrylic acid.

Further, in the present invention, a hydrophilic or hydrophobicunsaturated monomer may be used besides an acrylic acid (salt). Monomersusable may include methacrylic acid, maleic acid, maleic anhydride,2-(meth)acrylamido-2-methylpropanesulfonic acid,(meth)acryloxyalkanesulfonic acid, N-vinyl-2-pyrrolidone,N-vinylacetamide, (meth)acrylamide, N-isopropyl(meth)acrylamide,N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate,methoxypolyethylene glycol (meth)acrylate, polyethylene glycol(meth)acrylate, stearyl acrylate, and their salts.

(b) Crosslinking Agent (Inner Crosslinking Agent)

In the present invention, in terms of the water absorbent properties,use of a crosslinking agent (i.e.; inner crosslinking agent) isparticularly preferable. The crosslinking agent is used in an amount ofpreferably 0.001 to 5% by mole, more preferably 0.005 to 2% by mole,still more preferably 0.01 to 1% by mole, and particularly morepreferably 0.03 to 0.5% by mole to the monomer excluding thecrosslinking agent, in terms of physical aspect.

Examples usable as the crosslinking agent are one or more ofpolymerizable crosslinking agents (with polymerizable double bond of theacrylic acid), reactive crosslinking agents (with a carboxyl group ofthe monomer), and crosslinking agents having both of these properties.Concrete examples are, as a polymerizable crosslinking agent, compoundshaving at least two polymerizable double bonds in a molecule such asN,N′-methylenebisacrylamide, (poly)ethylene glycol di(meth)acrylate,(polyoxyethylene)trimethylolpropane tri(meth)acrylate,poly(meth)allyloxyalkanes, etc. Further, examples of the reactivecrosslinking agent are covalent-binding crosslinking agents such aspolyglycidyl ether (ethylene glycol diglycidyl ether or the like), polyalcohols (propanediol, glycerin, sorbitol, etc.), and ion-bindingcrosslinking agents such as polyvalent metal compounds of aluminum orthe like. Among these crosslinking agents, in terms of water absorbentproperties, polymerizable crosslinking agents (with the acrylic acid),particularly, acrylate type, allyl type, and acrylamide typepolymerizable crosslinking agents are preferably used.

(c) Neutralizing Salt

Preferable examples as a basic substance to be used for neutralizationof the acrylic acid may include monovalent bases such as alkali metalhydroxides such as sodium hydroxide, potassium hydroxide, and lithiumhydroxide etc., and alkali metal (hydrogen) carbonates such as sodium(hydrogen) carbonate, potassium (hydrogen) carbonate, etc. Particularly,in terms of decrease of the residual monomers, neutralization into analkali metal acrylate especially with sodium hydroxide is preferable.The preferable conditions or the like in these neutralization treatmentsare exemplified in International Publication No. 2006/522181 and thedisclosed conditions are applicable for the present invention. Theneutralization temperature is preferably in a range of 10 to 100° C.,more preferably in a range of 30 to 90° C. The neutralizationtemperature is properly determined in this range, and a neutralizationmethod described below is preferable to decrease the residual monomers.

(d) Concentration of Monomer

Monomers may be polymerized generally in an aqueous solution. The solidcontent is generally 10 to 90 mass %, preferably 20 to 80 mass %, morepreferably 30 to 70 mass %, and particularly preferably 35 to 60 mass %.The polymerization may be carried out in the form of a slurry (waterdispersion liquid) exceeding the saturated concentration; however: interms of physical properties, it is carried out in an aqueous solutionwith the saturated concentration or lower.

(e) Other Monomer Components

The aqueous unsaturated monomer solution may contain a water-solubleresin or a water absorbent resin such as starch, polyacrylic acid(salt), or polyethylene imine, in combination with a monomer, in anamount of, for example, 0 to 50 mass %, preferably 0 to 20 mass %,particularly preferably 0 to 10 mass %, and most preferably 0 to 3 mass%. The solution may further contain a various kinds of foaming agents(carbonates, azo compounds, air bubbles, etc.), surfactants, oradditives described below, in an amount of, for example, 0 to 5 mass %and preferably 0 to 1 mass % to improve the various physical propertiesof a water absorbent resin or particulate water absorbent agent to beobtained. A graft polymer obtained by using other components (e.g.,starch-acrylic acid graft polymer) or a water absorbent resincomposition is also generically referred to as a polyacrylic acid(salt)-type water absorbent resin in the present invention.

As the additive, a chelating agent, hydroxycarboxylic acid, or areducing inorganic salt may be added, and it may be added to the waterabsorbent resin in such a manner that the amount thereof is preferably10 to 5000 ppm by mass, more preferably 10 to 1000 ppm by mass, stillmore preferably 50 to 1000 ppm by mass, and particularly preferably 100to 1000 ppm by mass. A chelating agent is preferable to be used.

The monomer is also preferable to contain a polymerization inhibitor.Examples of the polymerization inhibitor include such as methoxyphenoletc., and the content thereof is preferably 200 ppm or lower, morepreferably 10 to 160 ppm, and still more preferably 20 to 100 ppm(relative to monomer).

(f) Polymerization Step (Crosslinking Polymerization Step)

Owing to the performance and the easiness of polymerization control, thepolymerization method may be carried out by spray polymerization ordroplet polymerization, but preferably, in general, it is carried out byaqueous solution polymerization or reverse phase suspensionpolymerization. The aqueous solution polymerization is preferably whichare conventionally difficult to control polymerization or improve thecoloring, and further preferably continuous aqueous solutionpolymerization. An especially preferable controlling method is acontinuous polymerization method for producing the water absorbent resinin a huge scale of 0.5 t/h or higher, further 1 t/h or higher, stillmore 5 t/hr or higher, and still further 10 t/hr or higher bypolymerization of an aqueous unsaturated monomer solution in one line.Consequently, the preferable continuous polymerization may includemethods described as continuous kneader polymerization (e.g. U.S. Pat.Nos. 6,987,151 and 6,701,41), continuous belt polymerization (e.g. U.S.Pat. Nos. 4,893,999 and 6,241,928, and US Patent Application PublicationNo. 2005/215734).

In addition, in the continuous polymerization, polymerization at a hightemperature starting (monomer at 30° C. or higher, 35° C. or higher,further 40° C. or higher, and particularly 50° C. or higher: the upperlimit is the boiling point), or a high monomer concentration (30 mass %or higher, 35 mass % or higher, further 40 mass % or higher, andparticularly 45 mass % or higher: the upper limit is the saturatedconcentration) can be exemplified as one preferable example.

The monomer stability is excellent in the present invention and thewater absorbent resin with white color can be obtained even by thepolymerization in such a high concentration and at such a hightemperature, and thus the effect is significantly exhibited in suchconditions. Preferable examples of high temperature initiatingpolymerization are described in U.S. Pat. Nos. 6,906,159 and 7,091,253etc. In the present invention, the monomer stability beforepolymerization is excellent and therefore, production in an industrialscale is made easy.

The polymerization can be carried out in atmospheric air; however, it ispreferable for coloring improvement to carry out the polymerization inan inert gas atmosphere of nitrogen or argon (e.g., oxygen concentrationof 1% by volume or lower) and also, the monomer is preferable to be usedfor polymerization after the dissolved oxygen in the monomer or thesolution containing the monomer is sufficiently replaced with an inertgas (e.g., less than 1 mg/L of dissolved oxygen). Even if such degassingis carried out, the monomer is excellent in the stability and thereforegelatinization before the polymerization does not occur and the waterabsorbent resin with higher physical properties and high whiteness canbe obtained.

(g) Polymerization Initiator

A polymerization initiator to be used for the present invention can beselected properly in accordance with the polymerization mode. Examplesof the polymerization initiator may include radical polymerizationinitiator such as a photodecomposition type polymerization initiator, aheat decomposition type polymerization initiator, and a redox typepolymerization initiator. The amount of the polymerization initiator maybe 0.0001 to 1% by mole preferably and more preferably 0.001 to 0.5% bymole to the monomer.

In the case of the amount of the polymerization initiator is large,coloring may possibly generate and in the case of the amount is low, itresults in increase of the residual monomer. Further, in the case of aconventional color-improve agent, it sometimes causes a negative effecton the polymerization; however, in the polymerization by the method ofthe invention, the coloring can be improved without causing any negativeeffect on the polymerization (such as previous time and the variousphysical properties) and therefore, it is preferable.

Examples of the photodecomposition type polymerization initiator mayinclude benzoin derivatives, benzyl derivatives, acetophenonederivatives, benzophenone derivatives, and azo compounds. Examples ofthe heat decomposition type polymerization initiator may includepersulfuric acid salts (sodium persulfate, potassium persulfate, andammonium persulfate), peroxides (hydrogen peroxide, tert-butyl peroxide,methyl ethyl ketone peroxide), azo compounds(2,2′-azobis(2-amindinopropane) dihydrochloride,2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, etc.). Amongthese radical polymerization initiators, persulfuric acid salts,peroxides, and azo compounds can be used as a photopolymerizationinitiator.

Examples of the redox type polymerization initiator may include theabove-mentioned persulfuric acid salts or peroxides in combination withreducing compounds such as L-ascorbic acid and sodium hydrogen sulfite.Further, combination use of a photodecomposition type initiator and aheat decomposition type polymerization initiator can also be exemplifiedas a preferable embodiment.

(2) Gel Shredding Step (Pulverization Step)

A hydrous gel-like crosslinked polymer obtained by polymerization(hereinafter, sometimes referred to as “hydrous gel”) may be dried as itis; however, it may be pulverized to be particulate (e.g., with a massaverage particle diameter of 0.1 to 5 mm, preferably 0.5 to 3 mm) duringpolymerization or after polymerization with a pulverizer (kneader, meatchopper, or the like) if necessary.

From the physical property aspect, regarding the temperature of thehydrogel at the time of gel pulverizing, the hydrogel is kept or heatedpreferably at 40 to 95° C. and more preferably 50 to 80° C. The resinsolid content of the hydrogel is not particularly limited; however, fromthe physical property aspect, it is preferably 10 to 70 mass %, morepreferably 15 to 65 mass %, and still more preferably 30 to 55 mass %.It is optional to add water, a polyhydric alcohol, a mixed liquid ofwater and a polyhydric alcohol, a solution obtained by dissolving apolyvalent metal in water, or their vapor, or the like. In the gelshredding step, a water absorbent resin fine powder or various kinds ofother additives may be kneaded.

(3) Drying Step

In order to accomplish a decrease in residual monomers, prevention ofgel deterioration (urea resistance), and prevention of yellowing in thepresent invention, the drying step is carried out via the gel shreddingstep after completion of the polymerization. The time until the start ofdrying via the gel shredding step is more preferable as it is shorter.That is, after being discharged out of the polymerization apparatus, ahydrous gel-like crosslinked polymer after polymerization starts to bedried preferably within 1 hour, more preferably within 0.5 hours, andstill more preferably within 0.1 hours (charged to a drier). In order toset the time within the range, shredding or drying is preferably carriedout directly without carrying out a storage step for the gel afterpolymerization. Further, to decrease the residual monomer and accomplishlow coloring, the temperature of the hydrous gel-like crosslinkedpolymer from completion of the polymerization to starting of the dryingis controlled preferably at 50 to 80° C. and more preferably at 60 to70° C.

The drying step provides a dried product having a resin solid content,which is calculated from a drying loss of the polymer (drying of 1 gpowder or particles at 180° C. for 3 hours) in an amount controlled tobe preferably 80 mass % or higher, more preferably 85 to 99 mass %,still more preferably 90 to 98 mass %, and particularly preferably 92 to97 mass %. The drying temperature is not particularly limited; however,it is preferably in a range of 100 to 300° C. and more preferably in arange of 150 to 250° C. To satisfy both of the high physical propertiesand whiteness, it is preferably that the drying temperature is 160 to235° C., more preferably 165 to 230° C. Further the drying time ispreferably within 50 minutes. If the temperature or the time is out ofthe above-mentioned range, it may possibly result in decrease of thewater absorption rate (CRC), increase of soluble matters (Extractables),and deterioration of whiteness index.

A various drying methods such as heat drying, hot-air drying, vacuumdrying, infrared drying, microwave drying, drying by a drum drier,azeotropic dehydration with a hydrophobic organic solvent, high humiditydrying using high temperature steam can be employed. It is preferablyhot-air drying with a gas with a dew point of preferably 40 to 100° C.and more preferably 50 to 90° C.

(4) Crushing and/or Classifying Step (Particle Size and Adjustment Sizeafter Drying)

The above-mentioned hydrous gel-like crosslinked polymer, the particlesize may be adjusted after the drying if necessary. The polymer ispreferably made to have a specified particle size to improve thephysical properties by surface crosslinking described below. Theparticle size can be adjusted properly by polymerization (particularlyreversed phase suspension polymerization), crushing, classification,granulation, and fine powder recovery. Hereinafter, the particle size isdefined by a standard sieve (JIS Z8801-1 (2000)).

The mass average particle diameter (D50) of the obtained water absorbentresin particles in the dried step before surface crosslinking isadjusted to be 200 to 600 μm, preferably 200 to 550 μm, more preferably250 to 500 μm, and particularly preferably 350 to 450 μm. It is morepreferable as the particles smaller than 150 μm are less, and theparticles are adjusted in a range of generally 0 to 5 mass %, preferably0 to 3 mass %, and more preferably 0 to 1 mass %. Further, it is morepreferable as the particles bigger than 850 μm are less, and theparticles are adjusted in a range of generally 0 to 5 mass %, preferably0 to 3 mass %, and more preferably 0 to 1 mass %. The logarithmicstandard deviation (σζ) of the particle size distribution is preferably0.20 to 0.40, more preferably 0.25 to 0.37, and particularly preferably0.27 to 0.35. Its measurement method may be a method described in, forexample, International Publication No. 2004/69915 and a method describedin EDANA-ERT 420.2-02 by using a standard sieve. The particle diameteris preferably applied also to the finally obtained water absorbent resinafter surface crosslinking.

In general if the particle size distribution is narrowed, that is, theupper and lower limits of the particle size are controlled to be narrow,the color becomes noticeable; however, the present invention is freefrom such color issue and is preferable. Accordingly, in the presentinvention, it is preferable to carry out a classification step to givethe ratio of particles with 150 to 850 μm of 90 mass % or more, morepreferably 95 mass % or more, particularly preferably 98 mass % (Theupper limit is 100 mass %) or more, after drying.

The bulk specific gravity of the water absorbent resin particles ispreferably 0.5 to 0.75 (g/cm³) and more preferably 0.6 to 0.7 (g/cm³). Ameasurement method thereof is described in detail in, for example, EDANAERT 460.2-02. In the case where the bulk specific gravity is notsatisfied, the stirring power index becomes difficult to be controlledor the physical properties may be lowered or powdering may be caused insome cases.

(5) Surface Treatment Step

The present invention provide a method for producing a polyacrylic acid(salt)-type water absorbent resin in a huge scale (particularly 1 t/hr)including a surface treatment step of adding and reacting a surfacetreatment agent to and with the dried water absorbent resin powder,wherein crosslinking reaction is carried out at a stirring power indexof 3 to 15 W·hr/kg in a transverse type continuous stirring apparatushaving stirring means including a feeding inlet and a discharging outletof a water absorbent resin and one or more rotary shafts having aplurality of stirring discs, and heating means after the addition of thesurface-crosslinking agent in the surface treatment step.

Herein, the stirring power index is defined as (stirring powerindex)=((power consumption of apparatus at the time surfacetreatment)−(power consumption at the time of idling)×average retentiontime)/(treatment amount per unit time×average retention time), and awater absorbent resin with high physical properties can be continuouslyand stably obtained even at the time of scale-up to an 1 t/hr or morelarge scale, based on a specified apparatus and specified parametersthereof (stirring power index).

The stirring power index can be easily calculated as described abovefrom the power consumption of apparatus at the time of surface treatmentand the power consumption at the time of idling. If this stirring powerindex exceeds 15 W·hr/kg, the physical properties (particularly, liquidpermeability) are deteriorated and on the other hand, if it is under 3W·hr/kg, the physical properties (particularly, water absorption againstpressure) are also deteriorated. The stirring power index is morepreferably in a range of 4 to 13 W·hr/kg, still more preferably 5 to 11W·hr/kg, particularly preferably 5 to 10 W·hr/kg, and most preferably 5to 9 W·hr/kg.

The control of stirring power index can be determined properly inconsideration of adjustment of the supply amount and discharge amount ofthe water absorbent resin, the particle size or bulk specific gravity ofthe water absorbent resin, the rotation speed and shape of theapparatus, the composition of the surface treatment agent, and theretention time, and the preferable conditions will be described later.

As an apparatus, it is essential to use a transverse type continuousstirring apparatus having stirring means including a feeding inlet and adischarging outlet of a water absorbent resin and one or more rotaryshafts having a plurality of stirring discs, and heating means. When itis not the apparatus, the physical properties at the time of continuousmanufacture and scale-up is deteriorated.

Hereinafter, the preferable surface treatment method and control methodof stirring power index will be described.

(5-1) Humidifying and Mixing Step

This humidifying and mixing step is a step of adding and mixing asurface-crosslinking agent to and with the water absorbent resin powderobtained through the polymerization step to the classification step.

(a) Surface-Crosslinking Agent

The present invention further includes a surface-crosslinking step afterdrying. The production method of the present invention is preferablyapplicable for a method for producing a water absorbent resin with highabsorption against pressure (AAP) and liquid permeability (SFC) andcontinuous manufacture in a huge scale (particularly 1 t/hr), andparticularly preferably applicable for high temperaturesurface-crosslinking of a water absorbent resin.

Treatment agents described in Patent Documents 1 to 19, particularlysurface-crosslinking agents, are used for the surface treatment in thepresent invention. From the viewpoints of physical properties at thetime of scale-up, covalent bonding surface-crosslinking agents are usedamong them, and preferably covalent bonding surface-crosslinking agentsand ion bonding surface-crosslinking agents are used in combination.

(Covalent Bonding Surface-Crosslinking Agent)

Examples of a surface crosslinking agent to be employed in the presentinvention may include various organic or inorganic crosslinking agents,and organic surface crosslinking agents are preferably used. From theviewpoints of physical properties of obtained water absorbent resin,preferable examples to be used as the surface crosslinking agent arepolyhydric alcohol compounds, epoxy compounds, polyamine compounds andtheir condensation products with haloepoxy compounds, oxazolinecompounds (mono-, di-, or poly-)oxazolidinone compounds, and alkylenecarbonate compounds. Particularly dehydration reactive crosslinkingagents containing polyalcohol compounds, alkylene carbonate compounds,and oxazolidinone compounds, which require a high temperature reaction,are usable. In the case where no dehydration reactive crosslinking agentis used, the physical properties may sometimes be inferior or thedifference of the effects of the present invention may sometimes be hardto be caused in some cases.

More concretely, examples are compounds exemplified in U.S. Pat. Nos.6,228,930, 6,071,976, and 6,254,990. Examples are polyalcohol compoundssuch as mono-, di-, tri-, or tetra-propylene glycol, 1,3-propanediol,glycerin, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol,1,6-hexanediol, sorbitol, etc.; epoxy compounds such as ethylene glycoldiglycidyl ether, glycidol, etc.; alkylene carbonate compounds such asethylene carbonate; oxetane compounds; and cyclic urea compounds such as2-imidazolidinone.

(Ion-Bonding Surface Crosslinking Agent)

Further, other than the above-mentioned organic surface crosslinkingagent, an ion-bonding inorganic surface crosslinking agent (crosslinkingagent derived from polyvalent metal) may be used to improve the liquidpermeability potential or the like. Examples usable as the inorganicsurface crosslinking agent may include divalent or higher, preferably,trivalent to tetravalent polyvalent metal salts (organic salts andinorganic salts) and hydroxides. Polyvalent metals to be used arealuminum, zirconium, etc., and aluminum lactate and aluminum sulfate areusable. These inorganic surface crosslinking agents may be usedsimultaneously with or separately from the organic surface crosslinkingagent. The surface crosslinking with polyvalent metals is exemplified inInternational Publication Nos. 2007/121037, 2008/09843, and 2008/09842,in U.S. Pat. Nos. 7,157,141, 6,605,673, and 6,620,889, in US PatentApplication Publication Nos. 2005/0288182, 2005/0070671, 2007/0106013,and 2006/0073969.

Further, other than the above-mentioned organic surface crosslinkingagent, a polyamine polymer, particularly, having a mass averagemolecular weight of about 5000 to 1000000 may, be used simultaneously orseparately to improve the liquid permeability potential and the like.Usable polyamine polymers are exemplified in U.S. Pat. No. 7,098,284,International Publication Nos. 2006/082188, 2006/082189, 2006/082197,2006/111402, 2006/111403, and 2006/111404 etc.

(The Use Amount)

The use amount of the surface crosslinking agent is preferably 0.001 to10 parts by mass and more preferably 0.01 to 5 parts by mass relative to100 parts by mass of the water absorbent resin particles. Water can bepreferably used in combination with the surface crosslinking agent. Theamount of water to be used is preferably in a range of 0.5 to 20 partsby mass and more preferably 0.5 to 10 parts by mass relative to 100parts by mass of the water absorbent resin particles. In the case ofusing the inorganic surface cross-linking agent and the organic surfacecrosslinking agent in combination, the agents are used preferably in arange of 0.001 to 10 parts by mass and more preferably 0.01 to 5 partsby mass, respectively.

Further, at that time, a hydrophilic organic solvent may be used and itsamount is in a range of 0 to 10 parts by mass and preferably 0 to 5parts by mass relative to 100 parts by mass of the water absorbent resinparticles. Still more, at the time of mixing a cross-linking agentsolution with the water absorbent resin particles, a water insolublefine particle powder, and a surfactant may coexist to an extent that theeffect of the present invention is not hindered, that is, in a range,for example, of 0 to 10 parts by mass, preferably 0 to 5 parts by mass,and more preferably 0 to 1 part by mass. The surfactant to be used andits use amount are exemplified in U.S. Pat. No. 7,473,739 etc.

(b) Mixing Apparatus

In the present invention, for mixing the surface treatment agent, acontinuous high speed rotation stirring type mixing apparatus.Especially, a transverse type continuous high speed rotation stirringtype mixing apparatus (e.g., FIG. 4) is preferable used. In addition,the surface treatment agent refers to the above-mentionedsurface-crosslinking agent, or a substituent thereof (e.g., a radicalpolymerization initiator such as a persulfuric acid salt and a monomer)and is also a concept including a solution or dispersion liquid thereof.The stirring speed is preferably 100 to 10000 rpm and more preferably300 to 2000 rpm. The retention time is within 180 seconds, morepreferably 0.1 to 60 seconds, and particularly preferably 1 to 30seconds.

(c) Temperature of Water Absorbent Resin Before Surface-Crosslinking

In the present invention, the temperature of water absorbent resinparticles (e.g., a water absorbent resin in which it is mixed with thesurface-crosslinking agent and then before it is introduced into thesurface-crosslinking step; the resin is also referred to as aparticulate water absorbent agent) to be supplied to thesurface-crosslinking step or to a transportation tube is preferably 30°C. or higher, more preferably 40° C. or higher, and still morepreferably 50° C. or higher. The upper limit thereof is preferably 100°C. and more preferably 95° C. Deterioration of the physical propertiesof the water absorbent resin particles (particulate water absorbentagent) can be suppressed by keeping the temperature of the particulatewater absorbent resin to be supplied to a transportation tube at aprescribed temperature or higher. Specifically, a significant effect iscaused on maintain of the physical properties such as saline flowconductivity (SFC).

(5-2) Heat Treatment Step

This heat treatment step is a step of heating the wet mixture of a waterabsorbent resin powder and a surface treatment agent solution mixed inthe humidifying and mixing step to cause surface-crosslinking reaction.

(a) Structure of Heating Apparatus

(Inclined Angle)

Heat treatment is carried out for the water absorbent resin after thesurface treatment agent is added to the stirring apparatus. Thetransverse type continuous stirring apparatus (e.g., FIG. 1) is anecessary apparatus. In terms of control of the stirring power index,the transverse type continuous stirring apparatus is preferable to havea downward inclined angle of 0.1 to 10°. The inclined angle is morepreferably 0.5 to 5° and still more preferably 1 to 4°. In the casewhere the inclined angle does not satisfy the range, the stirring powerindex becomes too high or too low and the physical properties of thewater absorbent resin may possibly be deteriorated in some cases.

(Aspect Ratio)

The aspect ratio (length of apparatus in movement direction/width ofapparatus of cross section to movement direction) of the transverse typecontinuous stirring apparatus is preferably 1 to 20. The aspect ratio ismore preferably 1.5 to 10 and still more preferably 2 to 5. The aspectratio is determined as the ratio of the vertical (movement direction)length and the transverse (perpendicular to the movement direction in aplane) length in the inside of the apparatus. In the case where theaspect ratio does not satisfy the range, the stirring power indexbecomes too high or too low and the physical properties of the waterabsorbent resin may possibly be deteriorated in some cases or the pistonflow property in the apparatus may be sometimes worsened and thestability of the performance may be worsened in some cases.

(Scraping Blades)

The transverse type continuous stirring apparatus is preferable to havescraping blades (the scraping blades are denoted, for example, by 90 ain FIG. 2). The scraping blades are described in Patent Document 31(JP-A No. 2004-352941). If the scraping blades are used, the stirringpower index can be controlled to be low and as a result, the physicalproperties of the water absorbent resin can be improved.

(Average Retention Time)

In terms of control of the stirring power index within the preferablerange, the average retention time of the water absorbent resin ispreferably controlled to be 0.05 to 2 hours. The average retention timeis more preferably 0.1 to 1 hour and still more preferably 0.2 to 0.8hours.

The average retention time measurement for the water absorbent resin inthe transverse type continuous stirring apparatus of the presentinvention will be described. The retention time in the apparatus (alsoknown as heating time or reaction time in the transverse type continuousstirring apparatus) is controlled by various factors such as effectivevolume of the apparatus (in the case where there is a stirring shaftarranged in the transverse direction, the effective volume refers to thevolume covering the upper most surface of a stirring disk as the apex),the amount of the water absorbent resin particles to be supplied,inclined angle, the rotation speed of the stirring shaft, the shape ofthe scraping blade, the bulk specific gravity of the water absorbentresin particles, the kind of the surface treatment agent, the height ofthe discharge bank installed in the discharge outlet of the transversetype continuous stirring apparatus. These factors considerably affectnot only the retention time but also the stirring power index. A methodof measuring the average retention time is carried out by actuallyoperating the apparatus under the condition in which the various factorsare fixed and measuring the mass of the water absorbent resin particlesremaining in the apparatus after stopping the operation of theapparatus. Alternatively, the average retention time can also bedetermined by unsteadily introducing a substance easy to be identified(for example, a compound containing sulfur) into the charging inlet ofthe apparatus as a tracer substance, tracing the concentrationfluctuation of the substance at the discharge outlet, obtaining theretention time distribution function, and carrying out calculationaccording to the retention time distribution function. As the tracersubstance, for example, a water-soluble sulfuric acid salt can be used.Further, as a concentration analysis method, there is a method oftracing the concentration fluctuation by measuring the intensity ratioof the characteristic x-ray of sulfur and a monovalent cation (e.g.,sodium) with EPMA, XMA, or the like in the case of a partiallyneutralized polyacrylic acid water absorbent resin. The retention timedistribution function and the average retention time are described indetail in “Introduction of Chemical Reaction Engineering (Hannou KogakuGairon)”, Hiroshi KUBOTA, issued by the Nikkan Kogyo Shimbun, Ltd.

(Surface Roughness)

The inside of the transverse type continuous stirring apparatus ispreferably smooth and the surface roughness (Rz) thereof is controlledto be 800 nm or lower preferably. The surface roughness (Rz) ispreferably 500 nm or lower, more preferably 300 nm or lower, still morepreferably 200 nm or lower, particularly preferably 185 nm or lower, andmost preferably 170 nm or lower. In the case where the surface roughness(Rz) of the inside of the transverse type continuous stirring apparatusdoes not satisfy the range, since the friction resistance with the waterabsorbent resin particles becomes high, the stirring power index becomestoo high and the physical properties may possibly be deteriorated.

The surface roughness (Rz) means the value of the maximum height of thesurface unevenness and is defined according to JIS B 0601-2001. Thelower limit of the surface roughness (Rz) is 0 nm; however, there is notso much difference in the case where it is about 10 nm, and about 20 nmis satisfactory.

From the above-mentioned viewpoint, a material for the transverse typecontinuous stirring apparatus is preferably stainless steel and morepreferably obtained by mirror finishing. The mirror finishing suppressesdamage on the water absorbent resin powder. Examples of the stainlesssteel to be used for the apparatus include SUS 304, SUS 316, and SUS316L.

The surface roughness (Ra) other than the surface roughness (Rz) is alsodefined according to JIS B0601-2001 and a preferable value thereof isalso the same as that of the surface roughness (Rz). The surfaceroughness (Ra) is preferably 250 nm or lower and more preferably 200 nmor lower. These surface roughness values may be measured according toJIS B 0651-2001 by a probe type surface roughness meter. The surfaceroughness can be applied not only for the heating apparatus but also forapparatuses before and after the heating apparatus, preferably, for acooling apparatus, a transportation pipe (particularly, a pneumatictransportation pipe) and a hopper, and the effect of improving physicalproperties by surface-crosslinking can be heightened.

(Rotary Shaft and Stirring Disk)

The transverse type continuous stirring apparatus has one or a pluralityof rotary shafts, preferably 2 to 10 rotary shafts, and particularly 2shafts. Further, the number of a stirring disk (e.g., in FIG. 2) or astirring blade may be determined properly in accordance with the size(capacity) of the apparatus, and it is preferably 2 to 100 disks andmore preferably 5 to 50 disks per one shaft.

(Periodical Shielding)

In terms of physical property stability and improvement bysurface-crosslinking, it is to periodically shield the stirring typemixing apparatus (preferably transverse type continuous high speedrotation stirring type mixing apparatus) and the transverse typestirring apparatus after the water absorbent resin and the surfacetreatment agent solution are mixed to be introduced into the transversetype stirring apparatus. The interval for the periodical shielding ispreferably 0.001 to 5 minutes, more preferably 0.005 to 1 minute, stillmore preferably 0.01 to 0.1 minutes, and particularly preferably 0.01 to0.05 minutes. Execution of the periodical shielding makes it possible tocarry out periodic introduction of the water absorbent resin into thecontinuous apparatus installed downstream (introduction of the waterabsorbent resin into a heating apparatus from a mixing apparatus or intoa cooling apparatus from a heating apparatus); that is, the introductionis turned on and off intermittently. In the case where no periodicshielding is carried out in the surface-crosslinking step, the physicalproperties of a water absorbent resin to be obtained may possibly bedeteriorated in some cases. The shielding ratio (the ratio of the timewhen the water absorbent resin is shielded from the continuous apparatusinstalled downstream) is preferably in a range of 1 to 80%, morepreferably 2 to 40%, still more preferably 5 to 30%, particularlypreferably 5 to 20%, and most preferably 5 to 10%, in terms ofstabilization of the physical properties (standard deviation). It issufficient that the water absorbent resin in the above amount range(e.g., 1 t/hr or more) is fed to a next apparatus even if the periodicalshielding is executed. For example, in the case of a rotary valve, theshielding interval is defined as the reciprocal number (minute) of therotation speed (rpm), and the shielding ratio is defined as a valuecalculated by dividing the theoretical rotation speed (rpm) per oneminute of the rotary valve needed for discharging a mixture (wet powder;a mixture of the water absorbent resin and the surface-crosslinkingagent solution) to be supplied to the continuous high speed mixingapparatus (theoretical rotation speed is obtained from the volume flowrate calculated from the volume per one rotation of the rotary valve,the mass flow rate of the mixture to be discharged, and the bulkspecific gravity) by actual rotation speed (rpm) of the rotary valve,and multiplying the calculated value by 100. The shielding ratio isspecifically defined as a value calculated by dividing the rotationspeed (rpm) per one minute of the rotary valve needed for discharging awet powder (a mixture of the water absorbent resin and thesurface-crosslinking agent) out of a mixing apparatus per unit time bythe actual rotation speed of the rotary valve.

The amount of the water absorbent resin retained by periodical shieldingis preferably 0 to 2 mass % and more preferably exceeding 0 and to 1mass % relative to the treatment amount. The volume per one rotation ofthe rotary valve may be determined properly and it is preferably 0.1 to0.001 [m³/lev (one rotation)], more preferably 0.2 to 0.002 [m³/lev],and still more preferably 0.1 to 0.01 [m³/lev]. In the case where theperiodic shielding is carried out or even when the periodic shielding isno carried out, when the continuous apparatuses (the mixing apparatus,the heating apparatus, and the cooling apparatus if necessary) areconnected, the distance from the outlet of an upstream apparatus and theinlet of an apparatus installed downstream is preferably 10 m orshorter. The distance is more preferably 5 m or shorter, still morepreferably 3 m or shorter, and particularly preferably 2 m or shorter.At the time of connecting these apparatuses, the apparatuses areconnected up and down; that is, the downstream apparatus is connected toa lower side of the upstream apparatus. A shielding apparatus of thewater absorbent resin particles may be installed between the upstreamapparatus and the downstream apparatus. The lower limit of the distancemay be determined properly in accordance with the sizes of theapparatuses or in a range in which a shielding apparatus described belowcan be housed. In the case where the distance is too large or theapparatuses are not connected up and down, the physical properties of awater absorbent resin to be obtained may possibly be deteriorated insome cases. In the case of connecting up and down, the mixing apparatus,the heating apparatus, and the cooling apparatus if necessary may beconnected up and down in this order. Connecting of the cooling apparatusmay be above or beside the heating apparatus.

A gate, a valve, a damper, a rotary feeder, a table feeder, or the likeis installed as a periodically shielding apparatus in a connecting partof the continuous apparatuses so that the periodical shielding can becarried out. Examples of the gate to be employed include a slide gate, aroller gate, a tainter gate, a radial gate, a flap gate, a rolling gate,and a rubber gate etc. Examples of the valve to be employed include aHowell-Bunger (fixed cone dispersion) valve, a hollow jet valve (amovable cone dispersion valve), a jet flow valve, a butterfly valve, agate valve (a partition valve), an orifice valve, a rotary valve (avalve for opening or closing by rotating a cylinder), and a Johnsonvalve (a valve for opening or closing by moving a conical valve bodyback and forth). These shielding apparatuses may be installed in anoutlet of the mixing apparatus (e.g., FIG. 4 and FIG. 5), or in an inletof the heating apparatus (e.g., FIG. 1), or in a middle part thereofafter the outlet of the mixing apparatus and the inlet of the heatingapparatus are connected. The shielding and connecting of the apparatusesare preferably carried out via a valve, particularly a rotary valve,among these shielding apparatuses. The size (it refers to diameter:however, in the case where the cross section is other than a circularshape, it is converted into the diameter of a circle with the samesurface area) of the valve may be selected properly and it ispreferably, for example, 1 to 100 cm in diameter and more preferably 10to 50 cm in diameter.

Each shielding apparatus is operated at less than 100% of the maximumtreatment amount (kg/hr; the maximum amount of a substance which can bepassed through the shielding apparatus per unit time). The operationcondition is preferably 5 to 95%, more preferably 10 to 90%, and stillmore preferably 20 to 80%. In the case where the operation condition ofthe shielding apparatus is out of the above-mentioned range, thephysical properties of a water absorbent resin to be obtained maypossibly be deteriorated in some cases and the performance may possiblybecome unstable. In the case where a rotary shielding apparatus such asa rotary valve is used, the rotation speed thereof may be determinedproperly and it is preferably 1 to 500 rpm (rotation/minute). Therotation speed is more preferably 5 to 200 rpm, still more preferably 10to 100 rpm, and particularly preferably 20 to 100 rpm. The maximumtreatment performance of the shielding apparatus may be determinedproperly and it is, for example, preferably 0.01 to 20 t/hr and morepreferably 0.1 to 5 t/hr.

(b) Operation Condition of Heating Apparatus

After the surface treatment agent is added to the stirring type mixingapparatus, preferably the transverse type continuous stirring apparatus,and the water absorbent resin and the surface treatment agent solutionare mixed, heat surface-crosslinking treatment is carried out. Thetransverse type continuous stirring apparatus is an apparatus necessaryfor heating treatment. The water absorbent resin is treated by heattreatment and is also treated by second heat treatment if necessary, andthereafter is treated by cooling treatment. The heating temperature(heat transfer surface temperature of a jacket or the like) is 70 to300° C., preferably 120 to 250° C., and more preferably 150 to 250° C.,and the heating time is preferably in a range of 1 minute to 2 hours.The heat treatment is carried out generally by a drier or a heatingfurnace. The present invention can provide a water absorbent resin withhigh whiteness even by high temperature heating or drying with air (hotblow) which conventionally causes intense coloration.

(Filling Ratio)

It is preferable to continuously supply the water absorbent resin insuch a manner that the filling ratio (volume ratio) of the transversetype continuous stirring apparatus with the water absorbent resin can be50 to 90%. The filling ratio is more preferably 55 to 85% and still morepreferably 60 to 80%. In the case where the filling ratio does notsatisfy the above-mentioned range, the stirring power index is hard tobe controlled, and the physical properties of a water absorbent resin tobe obtained may possibly be deteriorated in some cases. The position at100% filling ratio is the apex part of a stirring disk of a rotary shaftas described above.

It is preferable to continuously supply the water absorbent resin insuch a manner that the mass surface area ratio of the water absorbentresin in the transverse type continuous stirring apparatus can be 100kg/m²/hr or lower. It is more preferably 90 kg/m²/hr or lower and stillmore preferably 50 to 70 kg/m²/hr. In the case where the mass surfacearea ratio does not satisfy the above-mentioned range, the stirringpower index is hard to be controlled and the physical properties of awater absorbent resin to be obtained may possibly be deteriorated insome cases.

Herein, the mass surface area ratio is defined by the followingequation.(Mass surface area ratio)=(Mass flow rate of water absorbent resin perunit time)/(Heat transfer area of apparatus)

In the case where the jacket surface of an apparatus trough is only forheat insulation, the mass surface area ratio is defined as follows.(Mass surface area ratio)=(Mass flow rate of water absorbent resin perunit time)/(Heat transfer area of stirring shaft and stirring disk ofapparatus)

(Rotation Speed and Reaction Time)

According to the present invention, uniform heating and mixing can becarried out by adjusting the stirring speed of the transverse typecontinuous stirring apparatus to 2 to 40 rpm. If it is lower than 2 rpm,the stirring becomes insufficient and on the other hand, if it is higherthan 40 rpm, a fine powder tends to be generated easily in some cases.The stirring speed is more preferably 5 to 30 rpm. The retention time inthe apparatus is, for example, 10 to 180 minutes and more preferably 20to 120 minutes. If it is shorter than 10 minutes, the crosslinkingreaction tends to be insufficient. On the other hand, if it exceeds 180minutes, the water absorption performance may possibly be deterioratedin some cases.

(Pressure Reduction)

In the present invention, it is preferable to set the inside of thetransverse type continuous stirring apparatus to slightly reducedpressure. “Pressure-reduced state” means barometric pressure lower thanatmospheric pressure. In addition, “degree of pressure reductionrelative to atmospheric pressure” means the pressure difference with theatmospheric pressure and is denoted as a positive (plus) value in thecase where barometric pressure is lower than atmospheric pressure. Forexample, in the case where atmospheric pressure is standard atmosphericpressure (101.3 kPa), the expression that “degree of pressure reductionis 10 kPa” means that barometric pressure is 91.3 kPa. In thisinvention, “degree of pressure reduction relative to atmosphericpressure” may also be referred to simply as “degree of pressurereduction”. In the case where pressure is not reduced, a water absorbentresin powder may possibly flow over the air intake port of the mixingapparatus and it is thus not preferable. Dust (ultrafine particles ofthe water absorbent resin or inorganic fine particles used if necessary)is removed from the water absorbent resin by slightly reducing thepressure and thus it is also preferable in terms of a decrease in dust.

From the viewpoint of improvement of the effect attributed to pressurereduction, the lower limit of the degree of pressure reduction ispreferably higher than 0 kPa, more preferably 0.01 kPa or higher, andstill more preferably 0.05 kPa or higher. Excess pressure reduction maypossibly remove even a necessary water absorbent resin powder besidesdust to the outside of the apparatus and it may possibly result in adecrease in yield. Additionally, from the viewpoints of suppression ofleap of a powder in the system and of suppression of excess cost for theexhaust system, the degree of pressure reduction is preferably 10 kPa orlower, more preferably 8 kPa or lower, still more preferably 5 kPa orlower, and particularly preferably 2 kPa or lower. The preferablenumeral range of the degree of pressure reduction may be selectedarbitrarily between the lower limit and the upper limit.

(Atmosphere)

The atmosphere in the transverse type continuous stirring apparatus maybe air, or an inert gas such as nitrogen for prevention of coloration orprevention of combustion, and steam may be added properly. Thetemperature and the dew point are determined properly, and theatmospheric temperature (defined as the gas temperature in the upperpart space of the apparatus) is preferably 30 to 200° C. and morepreferably 50 to 150° C. The dew point is preferably 0 to 100° C. andmore preferably 10 to 80° C.

(5-3) Cooling Step

The cooling step is a step carried out arbitrarily after the heatingtreatment step. In the case where a dehydration reactive crosslinkingagent such as a polyhydric alcohol compound, an alkylene carbonatecompound, and an oxazolidinone compound is used as thesurface-crosslinking agent, it is preferable to carry out the coolingstep.

A cooling apparatus used in this cooling step is not particularlylimited and a transverse type continuous stirring apparatus to be usedin the above-mentioned heating treatment may be used and also, a biaxialstirring and drying apparatus in which cooling water is circulated inthe inside of the inner wall and other heat transmission faces and whichis exemplified in the Patent Document 41 (U.S. Pat. No. 7,378,453) maybe used. The temperature of the cooling water is controlled to be lowerthan the heating temperature in the surface treatment step andpreferably 25° C. or higher and lower than 80° C. In the presentinvention, the surface treatment reaction by heating can be controlledby a cooling apparatus arbitrarily installed and the physical propertiesof the water absorbent resin can be improved. Examples of the coolingapparatus to be suitably used may include cooling apparatusesexemplified in Patent Document 41 for mechanical stirring (optionallycombined with stirring by gas current) and also cooling apparatuses forstirring and mixing by stirring by vibrations and stirring by gascurrent in combination. Herein, it is preferable to carry out theperiodic shielding for the inlet of the cooling apparatus (connected tothe outlet of the heating apparatus) and further for the outlet of thecooling apparatus.

The cooling step is carried out preferably in the connected transversetype continuous stirring apparatus (e.g., FIG. 3). The stirring powerindex of the cooling apparatus is preferable 3 to 15 W·hr/kg, morepreferably 4 to 13 W·hr/kg, still more preferably 5 to 11 W·hr/kg,particularly preferably 5 to 10 W·hr/kg, and most preferably 5 to 9W·hr/kg. The pressure reducing is similarly carried out preferably inthe case of the heating step, and the periodic shielding is alsosimilarly carried out preferably in the case of the heating apparatus.Herein, the stirring power index (4 to 13 W·hr/kg, more preferably 5 to11 W·hr/kg, particularly preferably 5 to 10 W·hr/kg, and most preferably5 to 9 W·hr/kg) of the heating apparatus described above (also known asheat treatment apparatus, heater) may be the same or different from thatof the cooling apparatus; however, in terms of physical properties, thestirring power index of the cooling apparatus (also known as chiller) ispreferable to be smaller. The stirring power index of the coolingapparatus is preferably in a range of 0.99 to 0.25 times, morepreferably 0.95 to 0.50 times, and particularly preferably 0.90 to 0.55times as high as that of the heating apparatus.

(5-4) Others (a) Number of Surface Treatment Apparatuses

In terms of improvement of the stirring power index and physicalproperties, the polymerization step may be carried out preferably bycontinuous belt polymerization or continuous kneader polymerization anda plurality of surface treatment steps are preferably carried out inparallel for the polymerization step.

In the production method of the present invention, in terms ofimprovement of physical properties and stabilization, thesurface-crosslinking step is carried out in 2 or more lines for 1 lineof the polymerization step. The 1 line in the present invention meansone system in which steps proceed from a raw material (monomer) untilwhen a polymer gel, a water absorbent resin (including a recovered finepowder product), a particulate water absorbent agent and a final productare obtained. In the case where the system is branched into two, it isreferred to as “2 lines”. In other words, “2 or more lines” means a modein which two or more apparatuses are arranged in parallel and operatedsimultaneously or alternately in a single step.

In the present invention, in the case where the respective steps arecarried out in 2 or more lines, the upper limit for each step is about10 lines, especially preferably 2 to 4 lines, still more preferably 2 to3 lines, and particularly preferably 2 lines. The physical properties ofa water absorbent resin to be obtained are improved by adjusting thenumber of the lines within the above range. From a viewpoint that in thecase where the number of lines (divisions) is large, no dividing effectis caused, and the operation becomes complicated, and also it is noteconomical in terms of the cost, it is particularly preferable tosimultaneously operate 2 lines, that is, 2 or more of the sameapparatuses (particularly two apparatuses) in parallel.

In the present invention, the polymer gel or the water absorbent resin,which is a dried product of the polymer gel, is divided into 2 or morelines in the steps after the drying step, and the ratio of amountsdivided may be determined for every step without any particularlimitation. For example, in the case of dividing into 2 lines, the ratiois preferably 4:6 to 6:4, more preferably 4.5:5.5 to 5.5:4.5, still morepreferably 4.8:5.2 to 5.2:4.8, and most preferably 5:5. Even in the caseof dividing into 3 or more lines, it is preferable that the ratio of themaximum amount and the minimum amount of n divided portions is withinthe above range. The dividing operation may be carried out in acontinuous manner or in a batch manner and the ratio of amounts dividedis defined in accordance with the average amounts for a prescribed time.

In the present invention, the number of lines in thesurface-crosslinking step is not particularly limited and the numberthereof may be selected arbitrarily; however, in consideration of theconstruction cost and running cost of a plant etc., 1 line or 2 linesare preferable and 2 lines are particularly preferable. That is, interms of physical properties, it is most preferable that thesurface-crosslinking step and preferably further the crushing step andthe classification step all have 2 or more lines (the upper limit iswithin the above range) for 1 line of the polymerization step.

In addition, in the case where a plurality of apparatuses are installedin parallel in place of one apparatus in the present invention, theapparatuses in parallel may properly be miniaturized. Even if thetreatment capacity of the apparatus is miniaturized into ½, the cost ofthe apparatus is not lowered to a half, however, in the presentinvention, installation of specified apparatuses in parallel improvesthe physical properties of an absorbent agent to be obtained anddecreases the ratio of the product out of the specification and it isthus found that it consequently results in a decrease in cost.

US Patent Application Publication No. 2008/0227932 discloses a techniqueof carrying out “polymerization in 2 lines” and the latter half in oneline; Patent Document 30 (US Patent Application Publication No.2007/149760) discloses a technique of “connecting in series” of astirring and drying apparatus and a heating apparatus forsurface-crosslinking; and WO 2009/001954 discloses a technique of“connecting in series” of belt polymerization apparatuses. In contrast,in the present invention, the physical properties are improved andstabilization is accomplished more than before by “arranging(substantially the same) apparatuses in parallel” in the specified stepafter completion of the polymerization step for one polymerizationapparatus.

(Dividing Means)

In order to carry out surface-crosslinking in 2 or more lines in thepresent invention, a dividing step is included, and a dividing step of aparticulate hydrous gel or a particulate water absorbent resin, which isa dried product of the gel, is preferably included, and a dividing stepof a particulate water absorbent resin is more preferably included.

A dividing method to be employed may be, for example, the followingmeans (a-1) to (a-3) for the particulate water absorbent resin afterdrying.

(a-1) A method for dividing the particulate water absorbent resin afterstorage in a hopper. Preferably, a quantitative feeder for a powder isused. As the quantitative feeder, a cycle feeder or a screw feeder isused suitably.

(a-2) A method for dividing the particulate water absorbent resin duringthe time of pneumatic transportation to a plurality of hoppers.

(a-3) A method for dividing the particulate water absorbent resin at thetime of dropping (e.g., free fall). In this case, a riffle divider, a3-way divider, or the like having hills and dams are used for thedividing. Additionally, a JIS sample reducing and dividing apparatus(riffle divider) has a structure partitioned into a large number ofsmall chambers in which a fed sample is distributed alternately to twodirections.

A dividing method to be employed for the polymer gel afterpolymerization may be, for example, the following means (a-4) to (a-6)or the combination thereof, and the polymer gel is supplied to thedrying step in parallel.

(a-4) A method for dividing the particulate hydrous gel obtained by akneader or a meat chopper at the time of dropping (e.g., free fall). Forthe dividing, a riffle divider, a 3-way divider, or the like havinghills and dams are used in the outlet of the kneader or the meatchopper.

(a-5) A method for dividing the particulate hydrous gel by aquantitative feeder.

(a-6) A method for cutting a sheet-like gel obtained by beltpolymerization.

Among them, it is preferable that at least the particulate waterabsorbent resin after drying is divided and in order for that, thepolymerization gel or the particulate dried product is divided.

A preferable value of the dividing ratio of the particulate waterabsorbent resin and the polymerization gel to be divided in the mode isas described above.

Among them, the means (a-1) to (a-3) are preferably employed and themeans (a-1) is more preferably employed in terms of the quantitativesupplying property.

(b) Hopper

In terms of the surface-crosslinking property in the present invention,a hopper is preferable to be used before and after thesurface-crosslinking. The hopper to be employed is more preferably ahopper with an inverse truncated pyramidal shape, an inverse circulartruncated conical shape, a shape formed by adding a square pillar withthe same shape as the maximum diameter part of an inverse truncatedpyramid to the pyramid, a shape formed by adding a cylindrical columnwith the same shape as the maximum diameter part of an inverse circulartruncated cone to the cone. The material thereof is not particularlylimited; however, a hopper made of stainless steel is preferable to beemployed and the surface roughness thereof is within the above rangepreferably. A preferable hopper and the shape thereof are exemplified inPCT/JP 2009/54903 and such a hopper is recommended.

(c) Transportation of Water Absorbent Resin Before and afterSurface-Crosslinking

Various kinds of methods may be employed as a method of transporting thewater absorbent resin before and after surface-crosslinking, andpreferably pneumatic transportation is employed. From a viewpoint thatthe excellent physical properties of the water absorbent resin particlesand/or water absorbent resin powder can be maintained stably and theobstruction phenomenon can be suppressed, dried air is preferable to beemployed as primary air and secondary air used based on the necessity(additional air for pneumatic transportation). The dew point of the airis generally −5° C. or lower, preferably −10° C. or lower, morepreferably −12° C. or lower, and particularly preferably −15° C. orlower. The range of the dew point is −100° C. or higher, preferably −70°C. or higher, and it is sufficient about −50° C. in consideration ofcost. Further, the temperature of the gas is 10 to 40° C. and morepreferably about 15 to 35° C. The adjustment of the dew point ofcompressed air to be used at the time of pneumatic transportation to theabove range can suppress a decrease in SFC, especially, at the time ofwrapping the water absorbent resin as a product and thus, it ispreferable.

Besides the use of the dried gas (air), heated gas (air) may be used. Aheating method is not particularly limited and a gas (air) may bedirectly heated using a heat source or the transportation part or pipemay be heated to indirectly heat a gas (air) flowing therein. Thetemperature of the heated gas (air) is preferably 20° C. or higher andmore preferably 30° C. or higher, and preferably lower than 70° C. andmore preferably lower than 50° C.

A method of controlling the dew point, a gas, preferably air, may bedried properly. Specifically, examples thereof include a method using amembrane drier, a method using a cooling and adsorption type drier, amethod using a diaphragm drier, or a method using these methods incombination. In the case where an adsorption type drier is used, it maybe of thermal regeneration manner, a non-thermal regeneration manner, ora non-regeneration manner.

(6) Other Steps

Besides the above-mentioned steps, as required, a recycling step of theevaporated monomer, a granulating step, a fine powder removing step, afine powder recycling step, etc. may be added. Further, in order toexhibit the effect of color stabilization over time, prevent geldeterioration, or the like, an additive described below may be used forthe monomer or the polymer thereof.

[3] POLYACRYLIC ACID (SALT)-TYPE WATER ABSORBENT RESIN (1) PhysicalProperties of Polyacrylic Acid (Salt)-Type Water Absorbent Resin

In the case the purpose is to use the polyacrylic acid (salt)-type waterabsorbent resin of the present invention for a sanitary material,particularly a paper diaper, it is preferable to control at least one ofthe following (a) to (e), further two or more including AAP, and stillmore three or more by the above-mentioned polymerization andsurface-crosslinking. In the case the followings are not satisfied, thewater absorbent resin sometimes fails to exhibit sufficient function inform of a high concentration diaper described below. The productionmethod of the present invention is more effective for producing a waterabsorbent resin attaining the following physical properties andparticularly for stabilizing the physical properties (narrowing thestandard deviation). That is, among the following physical properties ofinterest, the production method of the prevent invention is preferablyapplied to a method for producing a water absorbent resin having a waterabsorption against pressure (AAP) of 20 g/g or higher for an aqueous 0.9mass % sodium chloride solution at a pressure of 4.8 kPa, a 0.69 mass %physiological saline flow conductivity (SFC) of 1 (×10⁻⁷·cm³·s·g⁻¹) orhigher, and a water absorption under no pressure (CRC) of 20 g/g orhigher, and more preferably applied to a production method within thefollowing range to improve or stabilize the physical properties.

(a) Water Absorption Against Pressure (AAP)

In order to prevent leakage in a diaper, the water absorption againstpressure (AAP) for an aqueous 0.9 mass % sodium chloride solution undera pressure of 1.9 kPa and that of 4.8 kPa is controlled to be preferably20 g/g or higher, more preferably 22 g/g or higher, and still morepreferably 24 g/g or higher as one example of means for accomplishingthe surface-crosslinking and the cooling step carried out thereafter.The AAP is more preferable as it is higher; however, in terms of thebalance between other physical properties and cost, the upper limit ofthe AAP is about 40 g/g at 1.9 kPa and about 30 g/g at 4.8 kPa. The AAPis shown as a value at 4.8 kPa unless otherwise specified.

(b) Liquid Permeability (SFC)

In some cases, in order to prevent a leakage from a diaper, the liquidpermeability under pressure, which is a flow conductivity SFC (definedin U.S. Pat. No. 5,669,894) to a 0.69 mass % physiological saline flowconductivity (SFC) is controlled to be 1 (×10⁻⁷·cm³·s·g⁻¹) or higher,preferably 25 (×10⁻⁷·cm³·s·g⁻¹) or higher, more preferably 50(×10⁻⁷·cm³·s·g⁻¹) or higher, still more preferably 70 (×10⁻⁷·cm³·s·g⁻¹)or higher, and particularly preferably 100 (×10⁻⁷·cm³·s·g⁻¹) or higheras one example of means for accomplishing the surface-crosslinking andthe cooling step carried out thereafter.

In order to more effectively improve the liquid permeability, especiallyto improve SFC to 25 (×10⁻⁷·cm³·s·g⁻¹) or higher, the present inventionis preferably applied for producing a water absorbent resin with highliquid permeability.

(c) Water Absorption Under No Pressure (CRC)

Water absorption under no pressure (CRC) is controlled to be preferably10 (g/g) or higher, more preferably 20 (g/g) or higher, still morepreferably 25 (g/g) or higher, and particularly preferably 30 (g/g) orhigher. The CRC is more preferable as it is higher, and the upper limitis not particularly limited; however, in consideration of balance withother physical properties, it is preferably 50 (g/g) or lower, morepreferably 45 (g/g) or lower, and still more preferably 40 (g/g) orlower.

(d) Amount of Water Soluble Components (Dissolve Amount)

The amount of water soluble components is preferably 0 to 35 mass % orlower, more preferably 25 mass % or lower, still more preferably 15 mass% or lower, and particularly preferably 10 mass % or lower.

(e) Residual Monomer

Using the above-mentioned polymerization as one example of achievingmeans, the amount of the residual monomer is adjusted to be generally500 ppm by mass or lower, preferably 0 to 400 ppm by mass, morepreferably 0 to 300 ppm by mass, and particularly preferably 0 to 200ppm by mass.

(2) Other Additives

Further, in accordance with the purpose, 0 to 3 mass % and preferably 0to 1 mass % of an oxidizing agent, an antioxidant, water, a polyvalentmetal compound, a water-insoluble inorganic or organic powder such assilica and metal soap, etc. as well as a deodorant, an antibacterialagent, a polymer polyamine, pulp, and thermoplastic fibers, etc. may beadded to the water absorbent resin.

(3) Purpose of Use

The purpose of use of the polyacrylic acid (salt)-type water absorbentresin of the present invention is not particularly limited; however, itis preferable to use the water absorbent resin for an absorbing articlesuch as a paper diaper, a sanitary napkin, an incontinence pad, or thelike. Particularly, the water absorbent resin exhibits excellentperformance in case of being used in a high-consistency diaper (onediaper in which a large amount of the water absorbent resin is used)that conventionally has a problem of malodor and coloring derived fromraw materials and particularly in the case of being used in a top layerpart of an absorbent body of the absorbing article.

The content (core concentration) of the water absorbent resin in theabsorbent body which may contain arbitrarily other absorbing materials(pulp fibers or the like) in the absorbing article is 30 to 100 mass %,preferably 40 to 100 mass %, more preferably 50 to 100 mass %, stillmore preferably 60 to 100 mass %, particularly preferably 70 to 100 mass%, and most preferably 75 to 95 mass % to exhibit the effect of thepresent invention. For example, in the case where the water absorbentresin of the present invention is used especially for an upper layerpart of an absorbent body in the above concentration, the waterabsorbent resin is excellent in the dispersion property of an absorbedliquid such as urine or the like owing to the high liquid permeability(liquid permeability against pressure) and therefore, an absorbentproduct such as paper diaper can efficiently distribute a liquid andimprove the amount absorbed by the whole of the absorbent product.Additionally, since the absorbent body keeps highly advanced whitecolor, an absorbent product with sanitary impression can be provided.

EXAMPLES

Hereinafter, the effects of the present invention will be made apparentby way of examples; however, the present invention should not beconstrued in a limited way based on the description of the examples. Inaddition, measurement methods for AAP, SFC, and the like in thefollowing description are as described above.

Production Example 1 Production of Water Absorbent Resin Particle (A)

A continuous production apparatus for a polyacrylic acid (salt)-typewater absorbent resin was used which was obtained by connectingrespective apparatuses for a polymerization step (static polymerizationon a belt), a gel shredding step (pulverization step), a drying step, acrushing step, a classification step, and a transportation step betweenthe respective steps and which could carry out the respective stepscontinuously. The production capacity of this continuous productionapparatus was about 1500 kg per an hour. Water absorbent resin particleswere continuously produced by using this continuous productionapparatus.

First, an aqueous acrylic acid sodium salt solution partiallyneutralized at 75 mol % was prepared as an aqueous monomer solution (1).The aqueous monomer solution (1) contained 0.06 mol % of polyethyleneglycol diacrylate (average n number=9) as an inner crosslinking agentrelative to the total mole number. The concentration of the monomer(partially neutralized acrylic acid sodium salt) in the aqueous monomersolution (1) was 38 mass %. The obtained aqueous monomer solution (1)was continuously fed onto a belt by a constant rate pump. Nitrogen gaswas continuously blown in the middle of the pipe used for the feedingand the oxygen concentration dissolved in the aqueous monomer solution(1) was adjusted to 0.5 mg/L or lower. In addition, the above-mentioned“average n number” means the average number of degree of methylene chainpolymerization in the polyethylene glycol chain.

Next, sodium persulfate and L-ascorbic acid were continuously mixed byline mixing with the aqueous monomer solution (1). In this line mixing,the mixing ratio of the sodium persulfate was adjusted to 0.12 g per onemole of the monomer and the mixing ratio of the L-ascorbic acid wasadjusted to 0.005 g per one mole of the monomer. The continuously mixedmaterial obtained by line mixing was supplied in a thickness of about 30mm to a flat plane steel belt having banks in both ends and subjectedcontinuously to static aqueous solution polymerization for about 30minutes to obtain a hydrous gel-like crosslinked polymer (1). Thehydrous gel-like crosslinked polymer (1) was finely shredded into about2 mm by a meat chopper with a hole diameter of 7 mm, spread on a movableporous plate of a continuous ventilating band drier in such a mannerthat the thickness thereof was adjusted to 50 mm, and dried at 185° C.for 30 minutes to obtain a dried polymer. Herein, the time taken fromthe outlet of the polymerization apparatus to the inlet of the drier waswithin 1 minute. The entire amount of the dried polymer was continuouslysupplied to a three-step roll mill, followed by crushing. The roll gapsof the three-step roll mill were 1.0 mm/0.55 mm/0.42 mm in this orderfrom the upper side. After the crushing, the crushed polymer wasclassified by a sieving apparatus having metal sieving nets with 850 μmmeshes and 150 μm meshes to obtain water absorbent resin particles (A)containing about 98 mass % of particles having a particle diameter of150 to 850 μm. The CRC of the water absorbent resin particles (A) was 35g/g and the bulk specific gravity thereof was 0.6 g/cm³.

Example 1

A water absorbent resin powder (1) was produced by using a continuousproduction apparatus involving a surface treatment step (wetting andmixing step, heating step, and cooling step), a particle regulationstep, and a transportation step connecting the respective steps,successively from the continuous production apparatus used in ProductionExample 1. That is, the classification step of Production Example 1 andthe surface treatment step in Example 1 were connected by thetransportation step.

The water absorbent resin particles (A) were pneumatically transportedto a temporarily storing hopper by pneumatic transportation (temperature35° C. and dew point −15° C.) from the classifying apparatus andcontinuously supplied at 1.5 t/hr via a constant rate feeder by a highspeed continuous mixing apparatus (Turbulizer/1000 rpm) and at the sametime a surface treatment agent solution (1) was mixed by spraying(wetting and mixing step). The surface treatment agent solution (1) wasa mixed solution containing 1,4-butanediol, propylene glycol, and purewater. The surface treatment agent solution (1) was mixed with the waterabsorbent resin particles (A) at a ratio of 0.3 parts by mass of1,4-butanediol, 0.5 parts by mass of propylene glycol, and 2.7 parts bymass of pure water relative to 100 parts by mass of the water absorbentresin particles (1) to give a mixture (1), a wet powder.

The obtained mixture (1) was then surface-treated by a transverse typecontinuous stirring apparatus (1) having a downward inclined angle of1°, an aspect ratio of 2.2, a paddle rotation speed of 13 rpm, tworotary shafts, and stirring disks having scraping blades, and a surfaceroughness (Rz) of the inner surface of 500 nm (heat treatment step). Atthat time, the inside of the apparatus (1) was suctioned by a suctioninggas discharge apparatus having a bag filter, and the inside pressure ofthe apparatus was reduced to 1 kPa. A rotary valve (periodicallyshielding apparatus) was installed in the inlet (connecting part withthe mixing apparatus) and outlet (connecting part with the coolingapparatus) of the apparatus (1). According to a previous test, theposition of a discharge bank which gave an average retention time ofabout 45 minutes and an average filling ratio of 75% was measured, andthe discharge bank was set at the position as measured. A heating sourceused for the surface treatment was pressurized steam at 2.5 MPa, and thetemperature of the mixture (1) in the apparatus was measured by athermometer installed near the discharge part of the transverse typecontinuous stirring apparatus (1), and the steam flow rate wascontrolled to carry out the heating in such a manner that thetemperature was adjusted to 198° C. The total surface area of thestirring disks and the stirring shafts was 24.4 m² and the mass surfacearea ratio calculated from the total surface area (heat transfer surfacearea) and the treatment amount was 61.5 kg/m²/hr. The stirring power atthe time of the surface treatment was 27.8 kW, the stirring power inidling was 13.5 kW, the average retention time was 45 minutes, and thestirring power index was 9.5 W·hr/kg.

Using a similar transverse type continuous stirring apparatus, it wascooled forcibly to 60° C. (cooling step). The stirring power index was8.5 W·hr/kg at this time.

The water absorbent resin was classified by a sieving apparatus toseparate the substance under 850 μm, and the substance on 850 μm(substance not passed through 850 μm) was again crushed and mixed withthe substance under 850 μm to give a water absorbent resin powder (1),as a particle size-regulated product which was the total amount of thesubstance under 850 μm.

The obtained water absorbent resin powder (1) had 30.5 (g/g) of CRC,29.8 (×10⁻⁷·cm³·s·g⁻¹) of SFC, 25.2 (g/g) of AAP, and 0.68 g/cm³ of thebulk specific gravity. The standard deviation of each physical propertyvalue was CRC: 0.16, SFC: 0.48, and AAP: 0.13. Additionally, thesephysical property values were average values of the measurement carriedout by sampling (5 points) every an hour until 5 hours were passed fromthe starting of the operation. The physical property values weredetermined in the same manner for the following Examples and ComparativeExamples. The results are shown in Table 1.

Comparative Example 1

A mixture (1), a wet powder, was obtained by mixing the surfacetreatment agent solution (1) with the water absorbent resin particles(A) as in Example 1 (wetting and mixing step).

The obtained mixture (1) was then surface treated by a horizontaltransverse type continuous stirring apparatus (2) (a downward inclinedangle of 0°) having an aspect ratio of 2.4, a paddle rotation speed of10 rpm, two rotary shafts equipped with stirring discs having scrapingblades, and also 1000 nm of surface roughness (Rz) in the inner face(heating treatment step). At that time, the inside of the apparatus (2)was suctioned by a suctioning gas discharge apparatus having a bagfilter, and the inside pressure of the apparatus was reduced to 1 kPa. Arotary valve (periodically shielding apparatus) was installed in theinlet (connecting part with the mixing apparatus) and outlet (connectingpart with the cooling apparatus) of the apparatus (1). According to aprevious test, the position of a discharge bank which gave an averageretention time of about 80 minutes and an average filling ratio of 44%was measured, and the discharge bank was set at the position asmeasured. A heating source used for the surface treatment waspressurized steam at 2.5 MPa as in Example 1, and the temperature of themixture (1) in the apparatus was measured by a thermometer installednear the discharge part of the transverse type continuous stirringapparatus (2), and the steam flow rate was controlled to carry out theheating in such a manner that the temperature was adjusted to 193° C.The total surface area of the stirring disks and the stirring shafts was55.7 m² and the mass surface area ratio calculated from the totalsurface area (heat transfer surface area) and the treatment amount was26.9 kg/m²/hr. The stirring power at the time of the surface treatmentwas 53.3 kW, the stirring power in idling was 24.4 kW, the averageretention time was 80 minutes and the stirring power index was 19.3W·hr/kg.

Using a similar transverse type continuous stirring apparatus, it wascooled forcibly to 60° C. (cooling step). The stirring power index atthat time was 17 W·hr/kg.

The water absorbent resin was classified by a sieving apparatus toseparate the substance under 850 μm as in Example 1, and the substanceon 850 μm (substance not passed through 850 μm) was again crushed andmixed with the substance under 850 μm to give a water absorbent resinpowder (2), as a particle size-regulated product which was the totalamount of the substance under 850 μm. The obtained water absorbent resinpowder (2) had 30.3 (g/g) of CRC, 22.1 (×10⁻⁷·cm³·s·g⁻¹) of SFC, 24.5(g/g) of AAP, and 0.70 g/cm³ of the bulk specific gravity. Table 1 showsthe analysis results.

Example 2

The same operation was carried out as in Example 1 to obtain a mixture(2), a wet powder, except that 0.01 parts by mass of an aqueous solutioncontaining 10 mass % of polyoxyethylene (number of methoxy group: 20)sorbitan monostearate, a surfactant, was added to thesurface-crosslinking agent solution (1) in Example 1. That is, themixture (2) contained further 0.001 parts by mass of the surfactant inthe mixture (1).

The obtained mixture (2) was then surface-treated by a transverse typecontinuous stirring apparatus (3) having a downward inclined angle of2°, an aspect ratio of 2.4, a paddle rotation speed of 10 rpm, tworotary shafts, and stirring disks having scraping blades, and a surfaceroughness (Rz) of the inner surface of 500 nm (heat treatment step). Atthat time, the inside of the apparatus (3) was suctioned by a suctioninggas discharge apparatus having a bag filter, and the inside pressure ofthe apparatus was reduced to 1 kPa. According to a previous test, theposition of a discharge bank which gave an average retention time ofabout 90 minutes and an average filling ratio of 50% was measured, andthe discharge bank was set at the position as measured. A heating sourceused for the surface treatment was pressurized steam at 2.5 MPa, and thetemperature of the mixture (2) in the apparatus was measured by athermometer installed near the discharge part of the transverse typecontinuous stirring apparatus (3), and the steam flow rate wascontrolled to carry out the heating in such a manner that thetemperature was adjusted to 193° C. The total surface area of thestirring disks and the stirring shafts was 55.7 m² and the mass surfacearea ratio calculated from the total surface area (heat transfer surfacearea) and the treatment amount was 26.9 kg/m²/hr. The stirring power atthe time of the surface treatment was 32.0 kW, the stirring power inidling was 24.4 kW, and the stirring power index was 5.1 W·hr/kg.

Using a similar transverse type continuous stirring apparatus, it wascooled forcibly to 60° C. (cooling step). The stirring power index atthat time was 5.2 W·hr/kg.

The water absorbent resin was classified by a sieving apparatus toseparate the substance under 850 μm, and the substance on 850 μm(substance not passed through 850 μm) was again crushed and mixed withthe substance under 850 μm to give a water absorbent resin powder (3),as a particle size-regulated product which was the total amount of thesubstance under 850 μm.

The obtained water absorbent resin powder (3) had 30.3 (g/g) of CRC,30.2 (×10⁻⁷·cm³·s·g⁻¹) of SFC, 25.1 (g/g) of AAP, and 0.66 g/cm³ of thebulk specific gravity. Table 1 shows the analysis results. The standarddeviation of each physical property value was CRC: 0.13, SFC: 0.47, andAAP: 0.11.

Comparative Example 2

A mixture (1), a wet powder, was obtained by mixing the surfacetreatment agent solution (1) with the water absorbent resin particles(A), as in Example 1 (wetting and mixing step).

The obtained mixture (1) was then surface treated by a horizontaltransverse type continuous stirring apparatus (4) (a downward inclinedangle of 0°) having an aspect ratio of 1.2, a paddle rotation speed of10 rpm, two rotary shafts equipped with stirring discs having scrapingblades, and also 500 nm of surface roughness (Rz) in the inner face(heating treatment step). At that time, the inside of the apparatus (4)was suctioned by a suctioning gas discharge apparatus having a bagfilter, and the inside pressure of the apparatus was reduced to 1 kPa asin Example 1. A rotary valve (periodical shielding apparatus) wasinstalled in the inlet and outlet of the apparatus (4). According to aprevious test, the position of a discharge bank which gave an averageretention time of about 20 minutes and an average filling ratio of 22%was measured, and the discharge bank was set at the position asmeasured. A heating source used for the surface treatment waspressurized steam at 2.5 MPa as in Example 1, and the temperature of themixture (1) in the apparatus was measured by a thermometer installednear the discharge part of the transverse type continuous stirringapparatus (4), and the steam flow rate was controlled to carry out theheating in such a manner that the temperature was adjusted to 205° C.The total surface area of the stirring disks and the stirring shafts was12.0 m² and the mass surface area ratio calculated from the totalsurface area (heat transfer surface area) and the treatment amount was125 kg/m²/hr. The stirring power at the time of the surface treatmentwas 28.0 kW, the stirring power in idling was 24.4 kW, the averageretention time was 20 minutes and the stirring power index was 2.4W·hr/kg.

Using a similar transverse type continuous stirring apparatus, it wasthen cooled forcibly to 60° C. (cooling step). The stirring power indexat that time was 1.5 W·hr/kg.

As in Example 1, the water absorbent resin was classified by a sievingapparatus to separate the substance under 850 μm, and the substance on850 μm (substance not passed through 850 μm) was again crushed and mixedwith the substance under 850 μm to give a water absorbent resin powder(4), as a particle size-regulated product which was the total amount ofthe substance under 850 μm. The obtained water absorbent resin powder(4) had 30.5 (g/g) of CRC, 20.5 (×10⁻⁷·cm³·s·g⁻¹) of SFC, 23.2 (g/g) ofAAP, and 0.62 g/cm³ of the bulk specific gravity. Table 1 shows theanalysis results.

Example 3

Water absorbent resin particles (B) were produced by changing thethroughput to be 3 t/hr from 1.5 t/hr in Production Example 1, further,the same operation was carried out as in Example 1 to obtain a mixture(3), a wet powder, except that the surface treatment step (wetting andmixing step, heating treatment step, and cooling step) in Example 1 wascarried out in 2 lines (two respective apparatuses using in therespective steps were arranged in parallel, and throughput was 1.5t/hr×2) for 1 line of the polymerization step (throughput: 3 t/hr), andthen a water absorbent resin powder (5) as a product was obtained. Thewater absorbent resin powder (5) obtained had 30.5 (g/g) of CRC, 29.5(×10⁻⁷·cm³·s·g⁻¹) of SFC, 25.2 (g/g) of AAP, and 0.67 g/cm³ of the bulkspecific gravity. Table 1 shows the analysis results.

Example 4

The same operation was carried out as in Example 3, except that thesurface-crosslinking step (each one apparatus as a wetting and mixingapparatus, a heating treatment apparatus, and a cooling apparatus) wascarried out in 1 line for 1 line of the polymerization step (3 t/hr).

The water absorbent resin particles (B) were continuously supplied at 3t/hr to a high speed continuous mixing apparatus (Turbulizer/1200 rpm)and at the same time a surface treatment agent solution (1) was mixed byspraying (wetting and mixing step). The surface treatment agent solution(1) was a mixed solution containing 1,4-butanediol, propylene glycol,and pure water. The surface treatment agent solution (1) was mixed withthe water absorbent resin particles (B) at a ratio of 0.3 parts by massof 1,4-butanediol, 0.5 parts by mass of propylene glycol, and 2.7 partsby mass of pure water relative to 100 parts by mass of the waterabsorbent resin particles (B) to give a mixture (4), a wet powder.

The obtained mixture (4) was then surface-treated by a transverse typecontinuous stirring apparatus (4) having a downward inclined angle of2°, an aspect ratio of 2.5, a paddle rotation speed of 10 rpm, tworotary shafts, and stirring disks having scraping blades, and a surfaceroughness (Rz) of the inner surface of 500 nm (heat treatment step). Atthat time, the inside of the apparatus (6) was suctioned by a suctioninggas discharge apparatus having a bag filter, and the inside pressure ofthe apparatus was reduced to 1 kPa. According to a previous test, theposition of a discharge bank which gave an average retention time ofabout 45 minutes and an average filling ratio of 75% was measured, andthe discharge bank was set at the position as measured. A heating sourceused for the surface treatment was pressurized steam at 2.5 MPa, and thetemperature of the mixture (4) in the apparatus was measured by athermometer installed near the discharge part of the transverse typecontinuous stirring apparatus (6), and the steam flow rate wascontrolled to carry out the heating in such a manner that thetemperature was adjusted to 200° C. The total surface area of thestirring disks and the stirring shafts was 46.5 m² and the mass surfacearea ratio calculated from the total surface area (heat transfer surfacearea) and the treatment amount was 64.5 kg/m²/hr. The stirring power atthe time of the surface treatment was 57.1 kW, the stirring power inidling was 24.3 kW, the average retention time was 45 minutes, and thestirring power index was 10.9 W·hr/kg. Using a transverse typecontinuous stirring apparatus having a similar shape, the waterabsorbent resin was forcibly cooled to 60° C. (cooling step). Thestirring power index at that time was 10.1 W·hr/kg.

The water absorbent resin was classified by a sieving apparatus toseparate the substance under 850 μm, and the substance on 850 μm(substance not passed through 850 μm) was again crushed and mixed withthe substance under 850 μm to give a water absorbent resin powder (6),as a particle size-regulated product which was the total amount of thesubstance under 850 μm. The obtained water absorbent resin powder (6)had 30.1 (g/g) of CRC, 28.5 (×10⁻⁷·cm³·s·g⁻¹) of SFC, 24.8 (g/g) of AAP,and 0.66 g/cm³ of the bulk specific gravity. Table 1 shows the analysisresults.

TABLE 1 Comparative Comparative Example 1 Example 1 Example 2 Example 2Example 3 Example 4 Water absorbnet resin particle (A) (A) (A) (A) (B)(B) Feed amount [t/hr] 1.5 1.5 1.5 1.5 1.5 × 2 lines 3 Surface treatmentagent solution (1) (1) (1) (1) (1) (1) 1,4-BD [Parts by mass] 0.3 0.30.3 0.3 0.3 0.3 PG [Parts by mass] 0.5 0.5 0.5 0.5 0.5 0.5 W [Parts bymass] 2.7 2.7 2.7 2.7 2.7 2.7 Surfactant [Parts by mass] none none 0.001none none none Mixture (Wet powder) (1) (1) (2) (1) (3) (4) Transverstype continuous stirring (1) (2) (3) (4) (5) (6) apparatus (1) × 2 linesInclined angel [°] 1 0 2 0 1 2 Aspect ratio 2.2 2.4 2.4 1.2 2.2 2.5Paddle rotation speed [rpm] 13 10 10 10 13 10 Surface roughness (Rz)[nm] 500 1000 500 500 500 500 Total surface area [m²] 24.4 55.7 55.712.0 24.4 46.5 Mass surface area ratio [kg/m²/hr] 61.5 26.9 26.9 12561.5 64.5 Average retention time [minute] 45 80 90 20 45 45 Averagefilling ratio [%] 75 44 50 22 75 75 Temperature of mixture near the [°C.] 198 193 193 205 198 200 discarge part Stirring power index [W ·hr/kg] 9.5 19.3 5.1 2.4 9.5 10.9 Water absorbnet resin powder (1) (2)(3) (4) (5) (6) CRC [g/g] 30.5 30.3 30.3 30.5 30.5 30.1 AAP [g/g] 25.224.5 25.1 23.2 25.2 24.8 SFC [×10⁻⁷ · cm³ · s · g⁻¹] 29.8 22.1 30.2 20.529.5 28.5 Standard deviation CRC 0.16 0.13 AAP 0.13 0.11 SFC 0.48 0.47Note) 1,4-BD: 1,4-butanediol/PG: propylene glycol/W: pure waterSurfactant: Polyoxyethylene (20) sorbitan monostearate (added in form ofan aqueous 10 mass % solution) [Parts by mass]: Amount per 100 parts bymass of water absorbent resin particles Water absorbent resin powder:Average value for 5 hours (sampling was done every one hour).

Example 5

The water absorbent resin powder (6) obtained in Example 4 waspneumatically transported by leading compressed air (dew point −15° C.and temperature 35° C.) into a pipe with a surface roughness (Rz) in theinner face of 200 nm, and wrapped. The SFC after the pneumatictransportation was 28.0 and the SFC decrease ratio was 1.8%. The bulkspecific gravity was 0.67 g/cm³.

Example 6

The same pneumatic transportation as in Example 5 was carried out,except that compressed air with a dew point of 20° C. was used. The SFCafter the pneumatic transportation was 27.2 and the SFC decrease ratiowas 4.6%.

CONCLUSION

As shown in Table 1, it could be understood that the physicalproperties, particularly liquid permeability SFC of a water absorbentresin after surface-crosslinking, is 25 (cm³·s·10⁻⁷/g) or higher bycarrying out at a stirring power index of 3 to 15 W·hr/kg.

It was also understood that pneumatic transportation using compressedair with a specified dew point was preferable from the results ofExample 5 and Example 6.

COMPARISON WITH CONVENTIONAL TECHNIQUES

The physical properties of a water absorbent resin were improved andstabilized (reduction in standard deviation) by carrying out periodicshielding and controlling the stirring power index in this application,as compared with those in Patent Documents 1 to 41, conventional surfacetreatment techniques. For improvements in the apparatus, for example,techniques of using specified mixing apparatuses for mixing apparatusesfor surface-crosslinking agents (Patent Documents 26 to 29) are knownand also techniques of heating apparatuses for carrying out reaction ofwater absorbent resins and surface-crosslinking agents (Patent Documents30 and 31) are known; however, such improvement techniques in theapparatus do not indicate the present application and correspond tocomparative examples in the present invention.

INDUSTRIAL APPLICABILITY

A water absorbent resin with high physical properties, particularly,high liquid permeability is provided by continuous production in a hugescale (e.g., 1 t/hr or more).

EXPLANATION OF REFERENCE NUMERALS

-   -   10: Driving apparatus    -   20: Transverse type drum    -   30: Raw material supply port    -   40: Heat medium inlet    -   40′: Heat medium inlet    -   45′: Heat medium outlet    -   50: Water absorbent resin discharge port    -   70: Rotary shaft    -   80: Stirring disk    -   80 a: Stirring disk    -   80 b: Stirring disk    -   81: Carrier gas introduction port    -   85: Gas discharge port    -   90: Scraping blade    -   90 a: Scraping blade    -   90 b: Scraping blade    -   100: Stirring apparatus (stirring means)

The invention claimed is:
 1. A method for producing a polyacrylic acid(salt)-type water absorbent resin, comprising a step of preparing anaqueous monomer solution of an acrylic acid (salt), a step ofcontinuously polymerizing the aqueous monomer solution, a step of finelyshredding a hydrous gel-like crosslinked polymer during or afterpolymerization, a step of drying the obtained particulate hydrousgel-like crosslinked polymer, and a surface treatment step of adding andreacting a surface treatment agent to and with the dried water absorbentresin powder, wherein crosslinking reaction is carried out at a stirringpower index of 3 to 15 W·hr/kg in a transverse type continuous stirringapparatus having stirring means including a feeding inlet and adischarging outlet of a water absorbent resin and one or more rotaryshafts having with a plurality of stirring discs, and heating meansafter the addition of the surface-crosslinking agent in the surfacetreatment step: wherein(stirring power index)=((power consumption of apparatus at the timesurface treatment)−(power consumption at the time of idling))×averageretention time)/(treatment amount per unit time×average retention time).2. The production method according to claim 1 further comprisingforcibly cooling the water absorbent resin powder by using a transversetype continuous stirring apparatus having a stirring power index of 5 to13 (W·hr/kg) after the heating treatment.
 3. The production methodaccording to claim 1, wherein the surface treatment step has two or morelines for one line in the polymerization step.
 4. The production methodaccording to claim 1, wherein a periodical shielding apparatus isinstalled in the inlet and/or the outlet of the transverse typecontinuous stirring apparatus to be used in the surface treatment step.5. The production method according to claim 1, wherein the transversetype continuous stirring apparatus to be used in the surface treatmentstep has a downward inclined angle of 0.1 to 5)(°).
 6. The productionmethod according to claim 1, wherein the transverse type continuousstirring apparatus to be used in the surface treatment step has a masssurface area ratio defined below of 100 (kg/m²/hr) or lower: wherein,(mass surface area ratio)=(mass flow rate of water absorbent resin perunit time)/(heat transfer area of apparatus).
 7. The production methodaccording to claim 1, wherein the transverse type continuous stirringapparatus to be used in the surface treatment step has an aspect ratioof 1.5 to
 5. 8. The production method according to claim 1, wherein thetransverse type continuous stirring apparatus to be used in the surfacetreatment step has scraping blades.
 9. The production method accordingto claim 1, wherein the transverse type continuous stirring apparatus tobe used in the surface treatment step has a surface roughness (Rz) inthe inner face of 800 (nm) or lower.
 10. The production method accordingto claim 1, wherein the bulk specific gravity of the polyacrylic acid(salt)-type water absorbent resin is 0.50 to 0.75 (g/cm³).
 11. Theproduction method according to claim 1, wherein a surfactant is added tothe polyacrylic acid (salt)-type water absorbent resin.
 12. Theproduction method according to claim 1, wherein the inside of thetransverse type continuous stirring apparatus is kept in slightlyreduced pressure.
 13. The production method according to claim 1,wherein a heat medium for the transverse type continuous stirringapparatus to be used in the surface treatment step is pressurized steam.14. The production method according to claim 1, wherein the surfacetreatment step is carried out by adding a dehydration reactivecrosslinking agent and heating treatment at 150 to 250° C.
 15. Theproduction method according to claim 1, wherein the surface treatmentstep is carried out by adding an organic surface-crosslinking agentand/or a polyvalent metal salt.
 16. The production method according toclaim 1, wherein the polymerization step is carried out by continuouskneader polymerization or continuous belt polymerization.
 17. Theproduction method according to claim 1, wherein the polyacrylic acid(salt)-type water absorbent resin has 25 (×10⁻⁷·cm³·s·g⁻¹) or higher offlow conductivity of physiological saline solution (SFC).